Trump has been feuding with the NFL since the 1980s

— -- President Donald Trump caused controversy in the NFL after suggesting that owners fire players who protest against the national anthem on Friday, but this isn't the first time the 45th president of the United States has had a beef with the league.

Trump's 1980s feud with the NFL

Trump's long-standing beef with the NFL dates back to the 1980s, when he was the majority owner of the New Jersey Generals in the United States Football League.

Trump acquired the New Jersey Generals after their 1983 inaugural season, and he soon went head to head with the NFL, poaching multiple top-ranked players from the dominant league. In 1985, less than two years after he became a team owner, he and a handful of other owners voted to switch from a spring to fall schedule.

The move was in an attempt to eventually force a merger with the NFL, according to The Associated Press.

Trump, described in the sports documentary "Small Potatoes: Who Killed the USFL?" as an "impatient real estate baron," had a vision that was "at odds with the league's founders."

He is often credited with killing the now-defunct professional league and played an integral role when the USFL filed an antitrust lawsuit against the NFL in New York, accusing the NFL of being a monopoly.

While jurors unanimously upheld that contention, they only awarded the USFL $1 in damages, a far cry from the $1.3 billion the league was requesting.

The "ironic verdict effectively forced the league out of business," according to the documentary.

Trump's unsuccessful bid for the Buffalo Bills

Although Trump had said after his USFL purchase that he'd rather "create something from scratch" than buy an NFL team, he still tried to become a member of the elite owners' club with a bid to purchase the Buffalo Bills.

In 2014, Trump said he entered a billion-dollar bid for the Buffalo Bills, though the AP reported that other accounts of the sale pegged the offer below a $1 billion.

"I did it a little tentatively," Trump previously said of the offer. "When I put the bid in for the Buffalo Bills, I always was a little concerned if the NFL would remember how I knocked the hell out of them."

Trump had been bested by a higher offer. Had he become an owner, he probably wouldn't have run for president, he told the AP last year. But, he said the bid for president was "more exciting" and "a lot cheaper."

Rex Ryan, who served as the Bills' head coach for the 2015 and 2016 seasons, said Sunday on ESPN that he was "p----- off" by Trump's criticism of players who protest the national anthem. Ryan had supported Trump and introduced him at an April 2016 rally in Buffalo during his campaign, the former coach said.

Trump's Twitter tirade against the NFL began in 2013

By the time Trump joined Twitter in 2009, he was no stranger to bashing the NFL. But, his criticisms finally became Twitter-official in March 2013, when he wrote "What is happening to the sport?"

Later that year, Trump made clear his views on former quarterback Tim Tebow's abilities as a player -- twice -- saying Tebow "deserves to be in the @nfl."

Trump alerted his followers in May 2014 that he was watching the NFL draft, saying that only a few of its attendees will become stars.

Trump's goodwill toward the NFL seemed to fizzle out completely as 2014 marched on.

In May 2015, Trump accused the NFL of targeting New England Patriots quarterback Tom Brady in Deflategate.

During the presidential election, Trump accused Democrats of conspiring to schedule debates at the same time as NFL games to allegedly minimize viewers. (The national political parties have no role in scheduling the dates for debates, according to Politifact.)

Over the weekend, Trump took to Twitter to echo the comments he made during a rally in Huntsville, Alabama, on Friday, when he suggested that NFL owners should fire players who protest against the national anthem.