Complex sentence: It began as a U.S. Army proposal to place a scientific satellite into orbit during the International Geophysical Year, however, that proposal was rejected in favor of the U.S. Navy's Project Vanguard.
Split into simpler sentences: It began as a U.S. Army proposal to place a scientific satellite into orbit during the International Geophysical Year. That proposal was rejected in favor of the U.S. Navy's Project Vanguard.