It's relevant because the Chinese and Russians can do that as well. Russia and China can take over Europe and Asia before the US even has a chance to blink if they wanted to. On top of that Russia can deploy forces into Western US in a matter of hours if they needed to (think Alaska down California as that use to be Russia's backyard). What you fail to understand is Russia and China's land mass allows them to deploy more quickly to places where it would take the US 6 months to deploy and then do military action or require a D-Day type event. So while you think the US can do this or that, it's only because US can't physically reach these areas without massive transport movement.
Neither Russia nor China have America's air carrier capabilities and don't even come close of America's navy. Russia does, but China does not, have America's submarine capabilities. If you think Russia might invade the United States through Alaska, then you'd have to think that the United States would also be able to invade Russia through Alaska.
The other thing to consider is that the US has bases all over the world (and is likely to get more, in places like Lithuania and Poland). With today's satellite capabilities, Russia can't build up forces in any border without the United States (and NATO) learning about it, and taking countermeasures. From US bases, carriers, and submarines, the US can rain long range missiles on any advancing ground force, and can bomb them from the air as well.
So, no, it's not so simple for Russia to start invading NATO territory without any credible reaction from the USA and NATO.
I think some posters here are grossly underestimating the US military. It still is the most powerful one in the world. The impression of weakness is because of scattered resources, focus on a fight against a multi-headed, non-convention, non-State adversary (stateless Islamist extremists), and both (paradoxically) involvement in too many places and a lack of political will to get involved in more skirmishes.
But whoa, a Russian invasion of a NATO ally or of the US homeland wouldn't be met with political unwillingness to act. It would be a whole different matter, no doubt about it. Besides, NATO and the US would be facing a State, not a multi-headed serpent. There still is no state in the world that can match the United States army in a direct confrontation.
If Russia invades a NATO member or attacks the US territory, you can be assured that suddenly everything will regain focus and the response will be overwhelming.
Also, do realize that in the next few years NATO is likely to upgrade and the European Union is likely to establish a Rapid Response Force.
NATO has twice as many foot soldiers as Russia, so not even that advantage (ground invasion using its extensive borders) would be so simple for Russia. Besides, foot soldiers are no longer the decisive asset in modern war.
Some misguided poster in another thread called me a warmonger and said I want to see NATO wage war against Russia.
Not at all. I'm no warmonger, but rather a pacifist. There is a big difference between debating something and thinking something is likely to happen, versus wishing that something to happen. I have no desire to see any of this happening. I hope deterrence will be sufficient.
I do believe in the idea of strong deterrence, because as a pacifist, I think it enhances the chances of maintaining peace.
If people start to perceive the US Military and NATO as weak, they might be tempted to put them to test.
That's why I'd rather see upgrades in capability, because if it becomes clear again that these forces are not weak, not paralyzed, and not negligible, then the opponents will be less likely to attack and peace will prevail.