We give them money, but are they grateful?
No, they're spiteful and they're hateful
They don't respect us, so let's surprise them
We'll drop the big one and pulverize them
The reference here is to U.S. foreign aid, given in money, food, and material (both military and non-military). In 2011, U.S. foreign aid amounted to $49.5 billion USD, a number only marginally smaller (in 2011 dollars) than it was in the early 70s, at the height of the Vietnam War.
The big one is a reference to the atomic bomb, which the U.S. famously dropped twice on Japan, effectively ending World War II and marking the prelude to the Cold War.