> US essentially defends the world, allowing other countries to spend money elsewhere.

The US doesn't “defend the world” by any stretch of the imagination. As for what it does commit to defending, well, other countries wouldn't let the US put the forces and facilities in their countries that the US wants there for global force protection of the US didn't also make a credible commitment to mutual defense; the US doesn't “defend the world” as a charitable service, it agrees to assist with the defence of select countries in order to contain others, secure access to key resources, and be in a position to project force to advance US interests anywhere in the world.