When you're next door to the bad guy, you have to take all scenarios into serious consideration. Including the US being unfaithful. Even during the Cold War, that was a serious question. Even in the US, we pondered it. Would the US risk direct attack in defense of Bonn, Paris and London? Or would we fold and break our agreements? Poland has been planning for that possible scenario for years. After all, they have a painful history of not being able to rely on allies for their defense.
If the EU has catching up to do, it's our fault. After WW2 and throughout the Cold War, it was in our interest to keep Europe's militaries weak and reliant on the US for defense policy. Our policy became their policy by default and there wasn't a hell of a lot they could do about it. When they did try to reflex their military muscles and act independently, Suez Crises, we slapped down France and the UK.
Fast forward to just after the Cold War - The US was less than pleased by the 1998 Saint-Malo declaration where France and the UK called for an independent European security and defense policy. Again, not in US interests for Europe to go it alone. So after all that, we're NOW bitching that Europe isn't pulling it's own weight in NATO? For decades we incentivized and arm twisted them NOT TO! 
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