What Major Lesson Did U

What major lesson did U.S.policy makers learn from the U.S.military actions in Korea,Vietnam,and Iraq?

A) Tanks are ineffective against large infantry formations.
B) The United States can succeed against guerrilla organizations with the help of China.
C) Democratic presidents are more likely to get the country into wars.
D) Public support for conflicts decreases if they last a long time without positive results.
E) The threat of nuclear weapons gives the United States a decisive military and diplomatic advantage.