Question

At what point could the United States no longer avoid involvement in World War II?

Answer 1

When Japan bombed Pearl Harbor - 7 December 1941.

Before that it was potentially avoidable, after it there was no chance.

Don't agree about Germany having to declare War - The Empire of Japan declared war on both the British Empire and the USA with the attack on Hawaii. America was allied to USSR and UK as a result of this act of war.

Note: it was not necessary for all Allies and all Axis powers to be at war - USSR did not declare War on Japan until August 1945 (after Hiroshima) - but nobody would dispute they were not involved in WW2

Hope this helps...