Short Answer 
In his *Infamy Speech* delivered after the attack on Pearl Harbor, President Franklin D. Roosevelt called for national unity and support for the war effort, emphasizing the need for defense against aggression and the protection of American values. He justified entering the war by describing the unprovoked nature of the Japanese assault and the threats it posed to American security and interests.
Step 1: Context of the Speech
President Franklin D. Roosevelt delivered the *Infamy Speech* to the Joint Session of Congress following the *attack on Pearl Harbor* by Japan. This speech was a response to the unexpected assault on a vital US military base, which united the nation in its call for action. It is also commonly referred to as the *Pearl Harbor Speech*, marking a significant moment in American history.
Step 2: Call to Support the War
In his address, Roosevelt made a passionate appeal for the American public and Congress to rally behind the war effort. He emphasized the importance of unity and resolve in facing this act of aggression. The speech sought to engage the audience by highlighting the following:
- Necessity of national defense against hostile forces
- Protection of American values and freedoms
- Encouragement of collective patriotism
Step 3: Explanation of Reasons for War
Roosevelt provided clear justifications for entering the war as a response to the attack. By detailing the circumstances surrounding the *Pearl Harbor attack*, he informed Congress and the nation about the seriousness of the situation. The reasons he highlighted included:
- Unprovoked nature of the Japanese assault
- Threat to American security and interests
- Necessity of standing against tyranny