Most advanced social democracies cannot defend themselves because voters have become so addicted to government benefits that when politicians are faced with the challenge of reducing military spending or transfer payments, they will always reduce military spending.
The social democracies of Western Europe have largely abdicated responsibility for their defense and would not exist in the current form without the U.S defense umbrella.
Had it not been for England and the U.S. Hitler would have conquered all of Europe. Japan would have conquered all of Southeast Asia.
Had the U.S withdrawn its defense umbrella from Europe after WWII, the Soviet Union would have taken Western Europe and would probably still exist.
A credible deterrence with an effective alliance system with responsible nations is the only way to avoid war with determined adversaries.
Never before in history had peace been so well-managed as by America following World War II; rebuilding its former enemies and setting them on the path to successful democracy and constitutional government, and managing alliances and creating international institutions and trading systems to promote world order and stand as a bulwark against the expansion of the Soviet Union.