What is the political motivation that makes US never invade Canada?
Motivation to ask this question: Why US have to buy natural resource from Canada instead of getting it for free?
This touches on a foundational question of political philosophy and international relations: peace & trade versus war & plunder.
It's not clear it has an answer, just a host of competing theories and interpretations. The realist theory is that it doesn't make sense to attack your friends or treat them harshly when you might need their help serving other goals. The liberalist theory is that free trade and peace are the natural and highest state of mankind, and warfare is itself a negative. The Marxist theory, to the extent it's even worth repeating, is that both countries are controlled by a cabal of rich and powerful plutocrats in collusion to strip the "Global South" of its resources and vitality.
In a deeper sense, you might simply ask why any individual human engages in trade rather than robbery and conquest.
First, there are a host of moral reasons why one might not want to steal and kill - including the close bonds of society, language, culture, family, and friendship between the US and Canada.
Second, trade is the normal way that humans interact and makes intuitive sense to most people, so naturally it tends to be the default mode (all human societies engage in exchange and barter).
Third, Canadians are not just pushovers and warfare may be costly, particularly as it encourages warfare and militarization of others who witness the cost of being unarmed, eventually resulting in spending more resources on implements of warfare than one can easily reap by conquest.
Fourth, those with a reputation of failing to honor contracts or engage in honest trade may find it difficult to undertake contracts to extract, transport, or dispose of the plundered resources.
Fifth, and perhaps most importantly, is that the US government doesn't engage in most of the trade with Canada, American companies and individuals do. So the costs are not directly borne by the US government, just by companies that some US politicians may wish to favor.
Of course, the decision would be made by politicians accountable to constituencies in the US. War with Canada to steal their resources would be deeply unpopular. The simplest answer is that nobody is invading Canada and taking their stuff because nobody really wants to.
It's the same motivation that makes most people not walk around punching people in the face and stealing their wallets. Just on a different scale.
And those all have political repercussions. At this point in time, not enough US politicians have decided that the ability to access Canada's natural resources directly warrants the repercussions.
However, if they keep sending Justin Bieber South of their border, things may change.
I would rather have added this as comments to the question and to NL7 answer but do not have the reputation..
There were invasions of Canada from the US, two that I can find are http://en.wikipedia.org/wiki/Invasion_of_Canada_(1775) and http://en.wikipedia.org/wiki/War_of_1812.
To add to NL7 answer, it should also be noted that Canada has been an English colony then has kept ties and is part of the commonwealth. Another issue with a war with Canada would have been an automatic war with the United Kingdom (as shown, in the other direction, during the world wars).
I'm fascinated by the notion that the costs an invasion and occupation of Canada would somehow provide more cost-efficient access to Canada's natural resources- especially the notion that it would suddenly be "for free".
An invasion - won't be free. An occupation - won't be free. Costs to extract, transport, refine etc. of Canadian resources - won't suddenly become free
NAFTA, however, means that the US is already getting access to Canadian markets without significant tariffs. In other words, you are already only paying transportation costs and profit margins on your purchase of raw materials. Materials that would still be need the same transport to purchasers in the 48 states, and you'd still be paying for a corporate markup - just to a different company. Or maybe not. Much of Canadian resources are already being mined or developed by US and Chinese interests anyway.
So, if you aren't saving much money on Canadian resources, and there are no real ideological differences that warrant an invasion, you are asking why invade a country for no real reason - and what's the cost of the invasion then? "Sorry ma'am, your son died invading north so that Home Depot can profit 10 cents a linear foot on lumber instead of Beaver Lumber." How's that going to sell?
And you think Canadian`s would roll over and accept occupation? That it would be cheap, easy, or quick? I think not...
Not saying that you might not be able to come up with reasons to invade Canada, just suggesting that your premise is overly simplistic at best.
This is a reply to Steve Melnikoff that I decided to upgrade to an answer in its own right. Mr Melnikoff wrote, "It's not true that war with Canada would now lead to war with the UK."
It bloody well would! I grant you that Canada moved to gradually become a sovereign nation from 1920 onwards, but it is still a member of the Commonwealth. If the UK were willing to go to war when the Falkland islands (pop.1,800 at the time) were invaded by Argentina, how much more willing should it be to do the same for the vastly larger population and territory of Canada.
Quite apart from the affront to her Majesty and the Commonwealth, Canada is a member of NATO and by treaty an attack on one NATO member is an attack on all. Even if the attacker is itself a member of NATO - we must assume from the premises of the question that the US has been expelled as a rogue by now.
Mexico would also have a strong motive to join in the alliance, for fear that it would be next. In fact many countries with no link to Canada might join in the alliance in order to deter all countries, not just the US, from setting a precedent of successful invasion. When Iraq invaded Kuwait in 1990 a great many countries allied to eject it for this reason.
Note that it might well be worthwhile for Canada and its allies to go to war even if the US is militarily stronger. We can hope that one reason for the US not invading any more recently than 1812 is that the result last time was the sack of Washington and burning down of the White House by Major General Robert Ross in 1814. The Commonwealth/NATO/Mexico/Concerned Bystander Countries Alliance can reasonably calculate that it does not have to defeat the US utterly, just make it very expensive in blood and treasure for the invasion to continue.
Which, of course, is also why it will never happen in the first place.