Prior to officially entering the war, the US government took a series of steps to sway public opinion towards supporting the Allied cause. The Roosevelt administration gradually intensified its rhetoric against Axis aggressions, with the President's speeches, such as the 'Arsenal of Democracy' and 'Four Freedoms', highlighting the ideological divide and the threat to American interests. The government also facilitated support through the Lend-Lease Act, which provided substantial aid to Britain and the USSR, indirectly aligning the US with the Allies. Propaganda efforts, including films and posters, played up the heroism of the Allies and villainy of the Axis, preparing the public for the possibility of entry into the war.

Between 1939 and 1941, American public opinion shifted significantly from favouring neutrality to supporting intervention. Initially, the majority of Americans were deeply isolationist, reflecting the trauma from World War I and the impact of the Great Depression. However, as Axis aggression intensified and Britain stood alone against Nazi Germany, sympathy for the Allies grew. Roosevelt's 'Four Freedoms' speech and the Lend-Lease Act further swayed opinion by framing the war as a fight for democratic values. The decisive event was the attack on Pearl Harbor, which instantly unified public opinion in favour of entering the war.

The Atlantic Charter was a pivotal declaration issued by the United States and Britain that outlined their vision for the post-war world, even though the US had not yet entered the war. Signed in August 1941, it was significant for several reasons. It solidified the US-British relationship, with both nations agreeing on key principles such as self-determination, freedom of the seas, and economic cooperation. This strengthened international perception of the US as a 'non-belligerent' supporter of the Allies. Moreover, the Charter laid the groundwork for the later formation of the United Nations and established a shared set of goals for the Allies beyond mere military cooperation, symbolising a moral and political commitment against Axis ideologies.

The Neutrality Acts of the 1930s were designed to limit US involvement in foreign conflicts by restricting trade and financial relations with nations at war. The Acts sought to avoid the factors that had contributed to the US entering World War I, such as the provision of loans to belligerents and the sinking of American merchant ships by German submarines. By banning the export of arms and munitions to warring nations, requiring non-military goods to be purchased on a 'cash and carry' basis, and forbidding loans and credits to these countries, the legislation aimed to keep the US impartial and reduce provocations that could lead to war.

Isolationists argued vehemently against US involvement in World War II based on several points. Firstly, they contended that the conflict was a European issue that did not warrant American bloodshed, drawing on the profound loss and perceived futility of World War I. They also feared the economic strain of warfare, recalling the debt and economic turmoil post-World War I. Additionally, there was a strong belief that the Atlantic and Pacific Oceans provided natural defenses, diminishing the need for direct engagement. Isolationists emphasised the importance of focusing on domestic issues, maintaining that the US should fortify its own defenses rather than intervene abroad.