What were specific reasons for President Truman delivering his speech on April 11, 1951?
I would think that one of the most overwhelming reasons for President Truman delivering his speech is to convince the American people of his need to dismiss General MacArthur. Within this explanation is a justification of the war in Korea. President Truman understood the larger need to explain the reasons of American action in Korea to the American people. His claims of wanting to avoid a "Third World War" as well as to "help the Republic of Korea against the Communist aggressors" are a part of this explanation. At the same time, President Truman reveals some of his fundamental beliefs in terms of the need to contain Communism wherever it spreads:
If history has taught us anything, it is that aggression anywhere in the world is a threat to peace everywhere in the world. When that aggression is supported by the cruel and selfish rulers of a powerful nation who are bent on conquest, it becomes a clear and present danger to the security and independence of every free nation.
Within this reason, President Truman is able to clearly state the case for American intervention wherever the Communist threat might present itself. This reason is critically important in the speech as it helps to raise the Truman foreign policy as one that morally views the spread of Communism as an evil that has to be stopped. The expanded rationale for this makes it clear in Truman's speech that disagreement with such a moral and political imperative cannot be tolerated. President Truman's speech ends with the idea that since General Mac Arthur did not evidently agree with such a moral and political imperative, he had to be released from his command. President Truman makes it clear in his speech that his rationale is to fully explain the moral and political imperatives behind his policy and thus how the need to relieve General MacArthur is a logical extension of this.