In many instances war is a unifying activity for a state. It makes people forget their own internal differences and shift their animosity onto a foreign group. In times of war, people consider following the status quo to be the patriotic thing to do, and anyone who offers criticism of the government or the war itself is at risk of being called a friend of the enemy.
Bourne has been largely correct in his statement that war coerces groups and individuals into conforming. This has been been true for much of American history, though the United States is more tolerant of dissent than other nations. The most notable war dissenters were the doves during the 1960s and 1970s who protested the Vietnam War. These peace groups became mainstream, and it even became popular to protest the war. There are some protests against American involvement in the Middle East; however, these are relatively muted compared to the protests over Vietnam.
Bourne's statement predicts the consequences and contradictions of American involvement in WWI. A nation that joined the war to "make the world safe for democracy" policed mail and made speaking out against the war a prison-worthy offense, with the most notable inmate being presidential hopeful Eugene Debs. In addition to governmental involvement to keep criticism to a minimum, the people themselves pushed their own patriotic agendas to appear loyal to the war effort. Peer-pressure led to people buying war bonds and enlisting in the draft. Leaders at the state and local level banned German-language literature and even conversations. There were even cases of lynch mobs attacking anyone who was considered not entirely in favor of the American war effort. The government engaged in little coercive efforts compared to the efforts taken by its own citizenry. This effort would be continued after the war against anything that resembled socialism, thus ending many progressive goals.