Unit cohesion and how it's been used to justify discrimination in the military

— -- When President Donald Trump tweeted his decision to bar transgender men and women from serving "in any capacity" in the U.S. military, he cited the "disruption that transgender in the military would entail." Hours later, Sarah Sanders, the incoming White House press secretary, reiterated that argument, saying that the presence of transgender troops "erodes military readiness and unit cohesion."

The idea that a category of people — from women to African-Americans and gays to now trans individuals — could destroy an army from within is an argument as old as the U.S. military itself. To critics, it is one that has not stood the test of time.

Shortly after George Washington became commander in chief of the Continental Army, he barred blacks — freed or enslaved — from military service, according to the Pentagon. He later relented, but only after the British offered freedom to any slave who joined their ranks and he was desperate for more troops. Despite their service, the new Congress passed legislation barring African-Americans from serving shortly after the war ended, but blacks would serve in segregated units in every U.S. conflict, including the War of 1812, the Civil War and World War I.

During World War II, the number of African-Americans in the military grew enormously, with nearly 700,000 serving in the Army in June 1944. That put growing pressure on the administration to desegregate the armed forces, but there was concern that doing so would hurt America's fighting power.

"The policy of the War Department is not to intermingle colored and white enlisted personnel ... To make changes would produce situations destructive to morale and detrimental to the preparation for national defense," reads a confidential War Department memo from January 1944. The War Department was the predecessor of the Defense Department.

It's an argument that resonated with wartime President Franklin D. Roosevelt, who, despite his progressive policies elsewhere, kept the armed forces segregated because he feared disrupting the military's spirit.

"In the present dangerous crisis, Negro Americans, as well as all other Americans, must make sacrifices to meet the emergency," he said in a statement in 1940. "At this time and this time only, we dare not confuse the issue of prompt preparedness with a new social experiment however important and desirable it may be."

After the war ended, President Harry Truman found the time was right, and in July 1948, he issued an executive order desegregating the military. But even then, he asked it be done slowly enough to not affect "efficiency or morale."

But perhaps what's most interesting is that after serving together, most troops dropped their opposition to integrating the military. According to a survey conducted by Truman's Committee on Equality of Treatment and Opportunity in the Armed Services in 1945, 64 percent of white troops had an unfavorable view of serving with African-Americans before they fought in an integrated unit, but 77 percent said they had a favorable view afterward.

Despite the finding, the same argument was used to defend the military ban on gays and lesbians and then the "don't ask, don't tell" compromise, which barred them from serving openly. President Bill Clinton signed the DADT legislation in November 1993.

"When men go into battle and fight, they don't fight for God, country, mom and homemade apple pie," Gen. Norman Schwarzkopf, a former commander of U.S. Central Command, told ABC's Barbara Walters in 1992. "It is the unit cohesion. It's your buddy on your left and on your right and your not wanting to let the unit down."

Letting gays serve openly would ruin that, he argued. "It does tend to polarize on the organization. It does tend to break down the cohesion," he said.

Sen. Sam Nunn, D-Ga., told ABC News essentially the same thing in 1993, saying, "The presence in military units by persons who, by their acts or by their statements, demonstrate a propensity to engage in homosexual acts, would cause an unacceptable risk to the high standards of morale, good order and discipline and unit cohesion that are absolutely essential to effective combat capability."

But to troops on the battlefield, that once again didn't ring true. After his two tours in Iraq, Army medic Sgt. Anthony Bustos told ABC News' Jonathan Karl in May 2010, "I know that none of my guys would have traded me for another medic, whether that medic be straight, gay, lesbian, whatever."

"Don't ask, don't tell" was repealed by legislation that President Barack Obama signed in December 2010.

The argument persists with female troops, with women viewed as disruptive to unit cohesion and even "distracting."

Women were first allowed to serve on the battlefield only as nurses during the Spanish-American War. With World War II and the advent of the Women's Army Corps, some positions, including cryptographers and pilots, opened up, culminating in Truman's signing the Women's Armed Service Integration Act in June 1948 and establishing a permanent presence of women in the military.

That expansion of opportunity continued until 1994, when the Clinton administration signed the Direct Ground Combat Definition and Assignment Rule, which prohibited women from serving in direct combat on the ground. The decision came after a 1992 report from the Presidential Commission on the Assignment of Women in the Armed Forces argued that women should not be allowed in such roles.

"Unnecessary distraction or any dilution of the combat effectiveness puts the mission and lives in jeopardy," the report said. "Risking the lives of a military unit in combat to provide career opportunities or accommodate the personal desires or interests of an individual, or group of individuals, is more than bad military judgment. It is morally wrong."

It wasn't until January 2016 that the Obama administration ordered all military occupations and positions be opened to any woman who meets the standards and qualifications.