Complex sentence: He proposed to first halt the North Korean advance and then counterattack, enveloping the North Koreans with an amphibious operation, but the timing was dependent on the arrival of reinforcements from the United States.
Split into simpler sentences: He proposed to halt the North Korean advance first and then to counterattack, enveloping the North Koreans with an amphibious operation. However, the timing was dependent on the arrival of reinforcements from the United States.