2020 is a year we'll never forget. Here's why.

To say that 2020 was a year for the history books is an understatement.

It was a time of struggle and change as an unprecedented health crisis, deeply contentious election and racial upheaval collided with daily life in the U.S. and around the world.

This is on top of a presidential impeachment trial and record-breaking climate events that stretched people and their leaders to the brink.

Above all, this year highlighted the divisions -- political, philosophical and ultimately physical, that separated us from one another and our normal routines in an attempt to curb the novel coronavirus.

It was a year of tremendous loss and sorrow that many experienced alone as well and a new normal that never really felt normal.

But, it was also a time of resolve, heroism and empathy as we learned to live our lives alone together.

As 2020 comes to a close, ABC News is looking back at some of the major events that have defined the year.

The novel coronavirus

The health crisis that rocked the U.S. and the world was first widely recognized early in the year, spreading through China and then impacting Europe.

On Jan. 21, the U.S. recorded its first case and nine days later, the World Health Organization declared a public health emergency. On Feb. 29, the U.S. saw its first COVID-19 death.

What seemed to many like a threat that could be contained at the U.S. border soon proved otherwise.

By the second week of March, thousands of Americans filled hospitals with severe COVID-19 symptoms and many states were forced to shut down non-essential businesses to curb the spread. Soon, hundreds were dying in the U.S. every day, many of whom were in New York City, which would become the epicenter of the pandemic in America.

President Trump declared a national emergency on March 13 and less than two weeks later, the U.S. led the world in the number of coronavirus cases, according to Johns Hopkins University, which has tracked the metrics of the pandemic.

Health officials such as National Institute of Allergy and Infectious Diseases director Dr. Anthony Fauci emphasized wearing face masks, social distancing, limiting crowds and avoiding common gestures like hugging and shaking hands, concepts that were foreign to many in the Western world.

What to know about the coronavirus:

• How it started and how to protect yourself: Coronavirus explained
• What to do if you have symptoms: Coronavirus symptoms
• Tracking the spread in the U.S. and worldwide: Coronavirus map

Despite the growing danger, Trump refused to issue a nationwide mask mandate and early on in the pandemic told Americans it was no worse than the flu. He was not alone in that assessment -- other officials said the virus was a "low risk" to Americans, although for most that changed over time.

By the end of May, more than 100,000 Americans were dead from the virus, and millions were unemployed.

Over the summer, states throughout the country saw a rise in the number of cases similar to New York’s and California’s spring surges after reopening their own economies, some in contravention to public health recommendations.

As fall approached, schools, sports and other businesses and services re-opened but cases continued to climb, especially in the Midwest. It was at this time that President Trump was hospitalized with COVID-19.

On Oct. 2, Trump was taken to Walter Reed Hospital after he was diagnosed with the virus and remained there for less than a week. Trump ultimately recovered from the virus after he received an experimental treatment.

In the fall, coronavirus cases soared to record levels and deaths and hospitalizations followed. The country averaged over 100,000 cases a day in December and reached over 200,000 a day on some occasions.

By comparison, India, which has four times the U.S. population, reported less than 30,000 cases and several hundred deaths a day in December, according to Johns Hopkins data.

The surging virus, which now took aim at most of the country, as opposed to largely targeting major cities like in the spring, threatened to overwhelm health systems and pushed already exhausted medical personnel to the brink.

But there was a sign of hope at the end of the year as two pharmaceutical companies revealed their COVID-19 vaccine was effective for a majority of patients and fast-tracked plans to get the first doses to the public by the end of December.

The first dozes of the vaccine that was manufactured by Pfizer were given to Americans on Dec. 14, following the U.K. the week before, but only a limited number of doses were available, mainly for healthcare workers and others in high-risk categories. A second vaccine, manufactured by Moderna, was given to American patients starting on Dec. 21.

Health experts predict it could be until late 2021 before the country sees any return to normalcy.

Politics

President Trump began his final year in office under the specter of an impeachment trial, the third time in U.S. history that such an event took place.

Trump was charged with abuse of power and obstruction of justice for allegedly attempting to coerce Ukrainian officials to provide election interference against then-Democratic Presidential candidate Vice President Joe Biden.

After a nearly three-week trial, the Senate voted along party lines on Feb. 5 to acquit the president on both charges.

It was at this time that the race among Democratic candidates for the presidential ticket heated up. Several candidates were strongly in the running in February, including Sen. Elizabeth Warren and Sen. Bernie Sanders, who won the New Hampshire primary.

However, Biden, who was lagging in the polls and early year races, picked up key endorsements such as Congressman Jim Clyburn, of South Carolina, which helped him turn the tide. After winning the South Carolina primary, the former vice president went on to win 10 states on Super Tuesday and five more states a week later.

By April, all of Biden’s opponents dropped out of the race and he became the presumptive nominee, just as the pandemic seemed to have halted the country. Biden shifted to virtual speeches from his home in Delaware during the spring and urged Americans to follow the health experts' guidelines and science.

When restrictions eased up in the early summer, Trump and Biden hit the road with their campaigns with COVID-19 leading the conversations.

The contrasts, however, couldn't have been starker. On one hand, Biden, wearing a mask and observing COVID restrictions, including avoiding large rallies, spoke about the need to contain the virus spread over restarting businesses.

Trump, who didn’t appear publicly in a mask until July, emphasized opening businesses, sports, schools and other large gatherings. He held his own large rallies, in spite of health orders in several states that prohibited such gatherings.

In addition to marked differences in campaigning, states relied in an unprecedented way on mail-in voting to help people avoid crowded polling sites and potential coronavirus exposure.

That change deepened the divisions in the electorate, fueled by Trump's unsubstantiated claims about potential widespread voter fraud absentee and mail ballots that were on the rise due to the virus. Record numbers of people also took advantage of early voting.

The 2020 election was historic in yet another way -- on Aug. 11, a week before the Democratic primary, the former vice president made a historic announcement about his running mate. Sen. Kamala Harris became the first Black, South Asian female vice presidential candidate and the two laid out their agenda for helping the nation during a virtual convention.

Biden and Trump came head-to-head for the first time in their first presidential debate on Sept. 29, where Trump lashed out at both his opponent and moderator Chris Wallace. A second town hall debate was canceled a few weeks later and the third debate on Oct. 22 went on with fewer fireworks from either candidate.

At this point, millions of Americans had already cast their votes through the mail or at early in-person polling sites. These ballots would be crucial to the outcome of the race.

As polls closed on Nov. 3, the presidential election could not be projected as several key states, including Pennsylvania, Arizona and Michigan, hadn’t completed their count of the absentee ballots.

Although Trump appeared to have an early lead in some of these states -with more Republicans choosing to vote in-person and on Election Day compared to Democrats- Biden gained votes throughout the week and by Nov. 7, ABC News declared him the winner.

While states began to certify the election, Trump and his team of lawyers rushed with a series of lawsuits seeking to overturn the results, claiming, without any hard evidence, that there was voter fraud. As of Dec. 14, Trump has not formally conceded the race, but Biden announced key members of his cabinet, including a COVID task force.

Race

America came face to face once again with its long-standing issues on race and policing this year following the killings of unarmed Black women and men.

On Feb. 23, two white men shot Ahmaud Arbery while he was jogging through the Satilla Shores, Georgia, police said.

The police and prosecutors didn’t initially file charges on the suspects, one of whom used to be an investigator for the district attorney’s office after they claimed the 25-year-old was shot while involved in a burglary.

For months, Arbery’s family and activists called for an investigation and questioned the initial reports of the shooting.

But a video of the killing released in May, which showed Arbery being chased by Gregory McMichael and his son Travis McMichael in a truck, changed that. The younger McMichael is seen fighting with Arbery over shotgun before the shots are fired.

Soon after the video's release, the McMichaels and William Bryan, the man who filmed the shooting, were all charged with several criminal counts including felony murder.

Less than a month after Arbery’s death, three undercover officers in Louisville, Kentucky conducted a late-night no-knock warrant at the home of Breonna Taylor, a 26-year-old EMT. The officers were investigating a suspected drug operation that was connected to Taylor’s ex-boyfriend.

Taylor was asleep with her boyfriend at the time, Kenneth Walker, when the officers attempted to open the door with a battering ram, according to court documents. Walker, a licensed gun owner with no criminal record, said he asked the men to identify themselves and fired a shot at the door.

The three officers fired 25 bullets, killing Taylor in her sleep. No drugs were found in the apartment.

For months Taylor’s family and activists around the world called for the three officers, Sgt. Jonathan Mattingly, Det. Myles Cosgrove and Det. Brett Hankison, to be criminally charged.

Protests, social media campaigns and other forms of activism went on for months as state officials investigated the shooting. In the meantime, the city banned no-knock warrants, fired Hankison and settled a wrongful death suit with the family.

In September, a grand jury indicted Hankison on three counts of wanton endangerment related to bullets that entered Taylor’s neighbors’ homes, but they didn't indict the other two officers, which sparked outrage and protests from Taylor's supporters. The investigation by the Kentucky state attorney general's office found Mattingly and Cosgrove were justified in opening fire since Walker shot first.

While Taylor and Arbery’s deaths helped spark this year's conversation on race, the 2020 Black Lives Matter movement reached critical mass following the Memorial Day death of George Floyd at the hands of Minneapolis police -- an incident caught on bystander video.

A convenience store clerk called Minneapolis police and alleged Floyd, a 46-year-old who worked as a local security guard, used a phony $20 bill. Four officers approached Floyd in his car and attempted to arrest him, but at one point one of the officers, Derek Chauvin, put Floyd on the street and placed his knee on his neck for over eight minutes.

The incident was filmed on several cameras and Floyd is seen screaming “I can’t breathe,” and crying out to his mother before going lifeless.

Chauvin was charged with second-degree murder and second-degree manslaughter, and the three other officers, Thomas Lane, Tou Thao and Alexander Kueng, were charged with aiding and abetting second-degree murder. The accused officers pleaded not guilty and are awaiting trial.

News of the death spurred protests across the country as Americans of all races and backgrounds called for police accountability and fair treatment of the Black community. A number of peaceful protests turned violent as officers, demonstrators and counter-demonstrators clashed, as well as instances of looting and property destroyed.

Floyd’s death and protests prompted elected officials to take steps to address the community’s concerns. In several cities, elected officials voted to reduce police budgets, review first responder policies and remove their Confederate related public memorials.

American corporations also addressed the movement during the summer by pledging solidarity and, in some cases, improving inclusion among their ranks.

Activists have vowed to keep the conversation going in 2021 and stop the discrimination against minorities from those in power.

Climate

In addition to a pandemic, racial unrest and political upheaval, environmental changes led to record-breaking natural disasters on both coasts in 2020.

High temperatures, powerful winds and low precipitation all helped to fuel explosive wildfires on the West Coast this spring and summer, according to officials.

Between January and November, California recorded 9,639 incidents that burned through more than 4.1 million acres, damaged over 10,000 structures and killed 31 people, according to California Department of Forestry and Fire Protection, or Cal Fire.

One of the incidents, the August Complex fire, was the largest wildfire in the state’s history, burning over a million acres and forcing thousands to evacuate their homes due to smoke issues, according to Cal Fire.

While firefighters battled the blazes on the West Coast, residents and first responders on the East Coast suffered week after week of dangerous coastal storms.

There were a record number 30 named tropical storms during the 2020 hurricane season, according to the National Oceanic and Atmospheric Administration.

A dozen storms made landfall, five of which struck Louisiana. Six of the storms were above Category 3 and one was a Category 5, according to NOAA.

Hurricane Laura was one of the most destructive storms to strike the country. The Category 4 storm hit the Gulf Coast states in late August with winds as high as 45 mph and a 6-foot storm surge.

Power was out for hundreds of thousands of households, whole neighborhoods were flooded and homes were destroyed. Thirty-nine people were killed by the storm.