In the cases of both World War I and World War II, the presidents of the United States asked Congress to declare war. In both cases, Congress complied.
President Woodrow Wilson was well-known as an opponent of American engagement in the ongoing war in Europe, hoping to remain neutral and continue to engage and do business with countries on both sides of the conflict. A few German actions, however, forced his, and Congress’s hand. For one, German submarines attacked commercial ships in the Atlantic Ocean, a tactic that sunk the ocean liner Lusitania and caused the deaths of 128 Americans. Another incident was Germany’s announcement at the start of 1917 of its intention to conduct unrestricted submarine warfare. The final straw was the discovery in early 1917 of German efforts at enlisting Mexico in the war against the United States.
The plot was published in American newspapers in March of that year, which compelled the president to ask Congress for an official declaration of war against Germany. The US joined on the side of the Entente, Great Britain and France. Congress, as noted, responded to President Wilson’s request with that declaration on April 4.
After the war, President Wilson tried to create the League of Nations with other European and global countries. The announcement of his “14 Points” intended to prevent the emergence of the conditions that led to World War I. However, the efforts to promote peace were essentially nullified by opposition in Congress, which enjoyed considerable support among the citizenry.
In the case of American entry into World War II, US citizens were against involvement in another of Europe’s wars. As with German actions preceding US entry into World War I, Congress’s hand was forced by Japanese actions. A shift in involvement happened after the surprise attack on US military installations in Hawaii and the Philippines. President Roosevelt anticipated the country’s eventual entry into the war on the side of Great Britain. He had supported the Allies through material shipments to supporting their efforts against Germany.
Congress, however, remained extremely reluctant to allow the United States to become mired in another war. The attack on Pearl Harbor, as noted, was the precipitating event that compelled President Roosevelt’s decision to ask Congress for a declaration of war. His request was granted on December 11, 1941.