Why are NATO and the US urging European allies to increase their defense spending?

European allies are being urged to increase defense spending because they cannot assume America's military presence and support will continue indefinitely. As Pete Hegseth emphasizes, the message to European allies is 'stark' - now is the critical time to invest in their own defense capabilities. This investment is necessary because European nations need to build self-sufficiency in defense matters rather than relying primarily on US protection. The urgency reflects changing geopolitical realities where Europe must take greater responsibility for its security to ensure stability even if American commitment fluctuates in the future.