Now you ask a fair question - why do we still have military bases in Germany?
For a moment, let's put aside our NATO commitment. Let's not talk about our promise to the people of Europe.
Instead, let's just talk about the direct military impact of those bases in Germany. There is an area in Germany called the Fulda Gap. It is the most geographically appropriate launching point of a Soviet incursion into Europe. Closing that door greatly complicates any attempt to attack Europe. Conversely, it is our most advantageous ground access point should we decide to invade Russia. So, the Soviets keep that door closed, as well. They commit troops and we commit troops - if we have a tank battalion there, they have to put a tank battalion there in response. If they have a tank battalion there, we have to put one there in response.
Sounds like a stand-off, doesn't it? EXCEPT - as you've pointed out, we spend about 14% of our federal expenditures on defense. Russia, on the other hand, spends about 21% on theirs - an unsustainable rate of expenditure. If you recall, Reagan's "Star Wars" initiative forced Russia to increase defense spending beyond its breaking point - and directly contributed to the collapse of the Soviet Union.
The same will hold true for the Putin regime. He will eventually go broke, spend too much trying to keep up, and be overthrown. Russia will go through another democratic evolution, and move closer to individual rights.
Now, back to NATO - we gave our word, we keep our word. Our support of NATO ensures the fealty of European countries - and just as importantly - European markets. Why do suppose it is that we don't have a military base in France, but so many in Germany?
Placing our troops in Germany - or anywhere else - is intrinsically cheaper than housing them in the US and sending them to the hot-spot of the day. What's the cost of a 50,000 acre military base in Germany compared to a 50,000 acre military base in California or South Carolina? What's the cost of maintaining 100,000 troops in South Korea compared to those same 100,000 in Nevada?