Russia may not start a war over gas, but they will certainly use gas as a weapon in any conflict. They are already doing this as a way of intimidating the western European nations to stay out of the current war in Ukraine.
You don't seriously think anyone in Europe is willing to get into a shooting war with Russia over Eastern Ukraine? They have to say things so as not to appear weak, but they are completely toothless (as is the US).
Your comment about the oceans protecting North America makes no sense at all. Those oceans didn't stop the US from invading Europe in 1917 and 1944, didn't stop the US from invading north Africa in 1943, didn't stop Japan from invading the Aleutian Islands or the south Pacific islands in 1942, didn't stop the US from retaking the south Pacific islands in 1942-1944, didn't stop the US from invading Korea in 1950, Vietnam in 1965, Iraq in 1991 and 2003, and Afghanistan in 2002. Hell, the oceans didn't even stop the British from burning down the White House in 1814. And someone needs to time-travel back to 1940 and tell Hitler that the English Channel and North Sea are insurmountable barriers to bombing the shit out of London. Do you really think the Russian military doesn't have the logistical capabilities to cross an ocean and occupy an undefended continent?
Let me guess, what history you studied was in the US, right?
OK, 1917 and 1944, in both cases, required a significant buildup of troops and resources on the spot before the conflict. US troops were massing, with all their stuff (and lots of the rest of us as well) for a couple of years before they actually crossed the channel.
The invasion of North Africa is not equal, in logistics or defenses, to a theoretical invasion of the US. The Germans and Vichy French in North Africa were at the far end of their own extended supply lines, the French didn't want to fight anyway. A very different prospect from invading a large country on its home turf.
Aleutian Islands and South Pacific - again, isolated locations with minimal defenses, and ditto their retaking, painful and bloody as they were. A very different prospect than an invasion of California or the Japanese home islands would have been.
Korea and Vietnam both had a local friendly force and a place to land resources without being under fire. And Korea, despite a massive effort and tremendous losses, was fought to a draw. Vietnam can only be considered a total disaster, not least because of the distance from the US, extended supply lines (and the fact it was a stupid idea in the first place). Neither of those are good comparitors to the USA as a defensible location other than to demonstrate how impossible it would be to invade the US when the most powerful countries in the world can't even manage a backwater pisspot war successfully.
Iraq - well, what can I say about that? Again, massive power imbalance at the start. And yet, extended supply lines and a committed local opposition meant that the US did not leave Iraq in any better shape than it arrived, and arguably much worse. Yes, the US invaded, but the logistical complexities and awfulness of it has meant you are bankrupting your nation to do it. Imagine trying to occupy something the size of the US against a determined resistance? Not going to happen.
The same can be said for Afghanistan - a massively unbalanced military operation has still resulted in a thin veneer of victory over what is essentially a draw. If the Pashtun/Taliban/whatever can prevent a total military victory by the most overpowered military since Ghengiz Khan, how do you think any force on the planet would do trying to occupy North America?
The British who burned the White House were actually Canadians in British uniforms who walked down to Washington. You're welcome. The US is in no danger of invasion by Canada, now or really ever. The ocean is not an issue, and any supply lines were between then Canadian colonies and Britain - and at the time Britain was the preeminent naval power on the globe.
As for Hitler and England, there is a substantial difference between the English Channel and the Atlantic or Pacific Oceans. It is possible to swim the Channel and planes can cross it in minutes. Even the Concorde took a couple hours to cross the Atlantic. And Hitler still couldn't manage the invasion, much as he wanted to. If he had it is very unlikely that the US would have been able or willing to participate or mount any counter-invasion of Europe a couple years later.
My point remains. It isn't like the US would be undefended with, say, half as many carrier groups (any one of which has enough firepower to defeat the next dozen or so nations). Or perhaps half as many bases, all over the world? Half as many first tier fighter planes, any one of which is vastly superior to the rest of the world's air forces? Fewer bombers?
At no point did I make the case that the US, Canada or anyone else should be undefended. I am only saying that the sheer logistics of invading and occupying a territory, across a huge ocean, in today's tech levels, make it utterly impossible. If China were to launch an invasion fleet, most or all of it would be at the bottom of the ocean before it got halfway (or even to Taiwan for that matter). We would all know about the preparations, which would have to be enormous and long-term. If Russia were to try, the same thing would happen - unless they invaded Alaska, in which case they would be stuck in Alaska with a lot of cold mountains to cross before they could actually accomplish much.
So who else is going to invade? Mexico - hardly. North Korea- the worst they could do would be a plastic minisub or something. There is and will be no credible invasion threat to the North American mainland in the next 50 years. ISIS? They are bad people, but they are not going to be invading the US and if they did they'd be cut to pieces. Not that they won't blow something up, but that does not constitute an invasion and occupation.
And there is nothing to say we couldn't focus our expenses on making the continent completely untouchable while still saving trillions.