My answer to If Europe has the resources to defend themselves then why doesn't the US just leave them to do it?
Answer by Ken Larson:
The U.S. is a member of the NATO alliance. That agreement involves the U.S. directly in the security of Europe.
In addition, there are many common weapons , ammunition and systems that involve NATO commonality. That arrangement creates not only security but economic reinforcement to the arms industries in Europe and America.