Depends on your goals, manifest destiny imperialist swine… 😉
But, it really is NOT a brilliant idea; REALLY (I am trying being polite, here — in fact it is a very stupid dumb-ass idea). Something that is best not done.
So why would the USA invade Canada? There are very few rational scenarios for the USA to do this, other than to acquire access to certain Canadian natural resources, e.g. oil, fresh water, Screech… That said, there are far easier ways to gain access to these resources with your largest trade partner, than to invade them. So, this would only happen for ideological reasons — say the Christian Right or the Alt-Right form the US government — i.e. irrational reasons.
Can the USA successfully invade and annex Canada? Yes, they can. They have the military power to do so. They can take the populous south of Canada along the border with the USA. The rest of the country would be problematic as it is very rural, and thinly populated. So, the USA could invade and occupy the south of Canada and annex it. They would be unable to successfully invade, occupy, and annex the entire country — though they could do so in name.
ONE: There would be a Resistance. Contrary to popular American opinion, it is not illegal to own guns in Canada. We just have more stringent gun control, we make it difficult to own a handgun, and we don’t permit ordinary citizens to own automatic weapons and anti-tank weaponry for home defence. But there are a lot of guns (rifles and shotguns) and the more rural populations have a higher rate of gun ownership than the urban areas.
In an American invasion, a significant portion of the Canadian Forces would be withdrawn to the north and then dispersed to engage in guerrilla warfare against the invaders. Also, those members of the Reserve would be called up and told to head north. Canada is an industrialised nation, not a country like Iraq; the USA cannot handle or crush the insurgents in Iraq, they would have less success against a Canadian insurgency/resistance. Oh, and the American myth that Canadians are a bunch of pacifist wimps — it is a myth. We prefer to resolve issues without violence, but will use violence if no other option is available, especially if we are the victim of an unprovoked attack (your American invasion would be an unprovoked attack). And you should ask the advice of people in your military who have served with Canadian military as to how much we are a bunch of wimps — I think that they would tell you different (they would also tell you how adaptable we are when we don’t have the fancy computer-aided weaponry, or that weaponry is inoperable, of modern warfare).
The USA may control the urban south, but there would be dissent, civil disobedience, and urban guerrilla units. All of which would make the control of the occupied south of Canada, very difficult. The usual manner of control by an occupier would be through harsh penalties for any resistance to the American authority — this would cow some of the populace, but embolden others. For the USA to effectively negate the Resistance in the urban centres, they would have to use extreme methods of repression (and so, enter into the realm of being a definite war criminal nation).
There would be support for the Canadian Resistance abroad. There would be international volunteers, and clandestine shipments of supplies and arms, and donations of money.
TWO: The USA would become a “rogue nation”, an international pariah state. You have just attacked and invaded and occupied and annexed a nation that has been your closest friend and ally. The optics on this are going to be very, very bad. Canada is also a member of NATO; if one member is attacked the rest of the alliance is supposed to come it their aid (I am unaware of there being any clause that feeds into American exceptionalism, that it is not a violation is the USA initiates an attack on a NATO member). So, this would trash NATO as far as a treaty and an alliance as the senior member has violated both. No member of NATO (okay, maybe, just maybe, the Baltic trio might) is going to support the USA and most members of NATO would quietly support the Canadian Resistance and support and recognise the Canadian government in exile (oh yes, there would be one). And if you think that Russia and China are not going to have something to say about this, or that there will not be consequences for the USA action of invading Canada — them you are in a fantasy-land.
This is just a very moronic and stupid idea that has little if no benefit to the USA. It is more idiotic than the other similar questions, “Can the US invade and conquer Mexico?” “Could the US successfully invade and occupy Australia?” “Should the US invade New Zealand?” All that will happen is that there will be tens of thousands of Canadians and Americans who lose their lives, billions of dollars will be spent on a war that the USA cannot win, the NATO alliance will fracture or be reformed with the new purpose to protect its members from aggression from any superpower (including the USA — you won’t be part of this reformed alliance), and the United States of America would lose all trust and confidence in the international community — a loss of reputation that America will never get back.