In this speech, Clay does not expressly say why he believes that the US went to war with Mexico. However, he very strongly implies that he knows. Clay implies that the US went to war in order to take territory from Mexico.
In the speech, Clay says that he thinks that no war should be declared without a statement of its aims. He says that Congress never stated the goals of the war when it declared war. Therefore, he is introducing a resolution in which Congress will explicitly state that the war is NOT for the purpose of getting more territory for the US or for the purpose of expanding slavery.