Question

How did the United States officials respond to Germany's aggression in the late 1930s?

Answer 1

The Treaty of Versailles was in effect. 
The US was in a position of Neutrality in European affairs. 
Once the Axis Powers were formed, German-Japanese Alliance, all treaties were nullified. 
Then one of The Axis Powers attacked a US Navy Base in Hawaii, it was on like donkey-kong after that. 
Wars are messed up (Credit to google)

Answer 2

By trading with them and complaining when they sank US shipping.