Complex sentence: The U.S. fears that the Phalanx Nations are trying to send terrorist spies to the U.S. and they take immediate action, although the U.S. never really goes to war with them until 50 years later.
Split into simpler sentences: The U.S. fears that the Phalanx Nations are trying to send terrorist spies to the U.S. and they take immediate action. Though, the U.S. never really goes to war with them till 50 years later.