why does Threau describe the mexican war as the work of compratively a few individual using the standing government as their tool?

Because the majority of America was opposed to the war. Abolitionists viewed the was as an attempt to expand slavehoding territory in the United States. The vast majority are not willing to stand up for what they believe, this is a recurring theme within the essay. "If one honest man would stand up..." So the few are doing what they want because the majority are too apathetic or too afraid to speak up.