Answer:
Was Franklin D. Roosevelt's (FDR's) speech following the attack on Pearl Harbor effective? Of course, it was.
Background
On Sunday, the 7th day of December 1941, the Japanese without formal warning launched a surprise attack on a United States base destroying thousands of lives and US military equipment and facility. It was a huge blow to the United States especially because peace talks were ongoing. During the trial at the International Military Tribunal for the Far East, the attack was ruled to be a War Crime.
Following the attack, Franklin D Roosevelt gave a speech in which he attributed the attack not only to Japan but to the trio states also known as the Axis Powers - Japan, Germany, and Italy. By the 8th day of December 1941, World War II was already on as Britain was also attacked.:
The Speech
In the speech following the war, Franklin D. Roosevelt (FDR) the then thirty-second President of the United States of America gave a speech addressing the world. In his speech, he referred to the date of the attack as one which will be remembered for its "infamy".
1. He also stated that regardless of how long it took, the United States of America will have absolute victory.
2. In another paragraph, FDR stated that America would make sure such a thing didn't happen again.
Statement two above was achieved because, on the second day of September 1945 after two successful atomic bombings which claimed an estimated number of one hundred and sixty-five thousand lives, Japan surrendered to the United States formally after it had already done so via the radio broadcast of Emperor Hirohito.
To ensure that statement one happened, America went to court at the International War Tribunal, and Japan was also found guilty of war crimes.
Based on the above, it may be concluded that FDR's speech was very effective. For even after he had died, United States continued with the war and achieved victory.