I really enjoyed this analysis from someone who has the military background to understand the situation, combined with his service in Ukraine. I'll cite the last section which is especially instructive:

Political Consequences of War: Insurgency, Mutiny, Civil War

The consequences of any decision to move to Greenland or Canada are grave for the people of the United States. The political turmoil that would follow would immediately constitute a Constitutional crisis as the Republican party would likely provide cover and surrender its role as a co-equal branch of government that could rein in such a wild idea.

On a global scale, the United States would become a pariah nation. Only Russia and a few hanger-on states in Asia and sub-Saharan Africa would view this as a positive event. Europe would have to make the terrifying decision of cutting off all trade with the United States. Both Canada and Denmark are NATO nations. Both would have no option but to invoke Article 5, calling for the collective defense of their lands. This would mean that the nation that founded NATO would be at war with the other 31 countries in the alliance.

Despite America having strategic nuclear weapons and dominant armed forces, the political turmoil would not be limited to the civilian world. Members of the Armed Forces, 43% of whom are minorities, principally people of color, and a few remaining loyal officers and enlisted, would likely view orders to attack our neighbors without aggression as an unlawful order. As many as 60% of the American Armed Forces would refuse to execute any operation to attack Canada. Military units, such as the 10th Mountain Division in Fort Drum, New York, are just a few miles from Ontario. On weekends, they often cross the border to Canada to drink and entertain with their families. An order to take their weapons and kill the Canadian Customs and Border police, city SWAT teams, Royal Canadian Mounted Police, and mobilizing Canadian Armed Forces as well as innocent civilians is not an order that any true American soldier would follow.

Trump would be made aware of this by his highly underqualified Secretary of Defense. Like the Russian invaders of Ukraine, orders would go out under the guise of “training exercises” and end with the seizure of Greenland and Canada. However, the other critical units, such as the 101st and 82nd Airborne Divisions, would be needed to take Montreal, Quebec City, Ottowa, Toronto, Edmonton, Regina, and Vancouver. Their mobilization for Greenland operation would provide cover for a blitz on Canada. The operations concept would be to “dare” the Canadians to open fire on them in hopes that the country would just “Lay back and let it happen.” Team Trump’s sycophants somehow seem to believe that the Canadians would surrender quickly and quietly.

They have never been to a hockey game.

If this all sounds vaguely familiar, it is because that is the exact plan Vladimir Putin had for his invasion of Ukraine. And like the invasion of Ukraine, the Canadian people would likely rise up as a body and fight to the death. The US may be able to seize a few airports and, over a few weeks, push heavy forces into Canada, but all this would do is send in a protracted paramilitary counterinsurgency. It is one thing to seize terrain and another to hold it. The American public would be confronted with video images of their sons and daughters being picked off one by one at night by Canadian soldiers, reservists, and citizens who volunteered as paramilitary partisans. Imagine Afghanistan, but where the opponents have some of the most advanced surface-to-air missile systems, shooting American helicopters down with impunity or IEDs improvised, explosive devices hidden in Beaver dams or moose carcasses. The very idea that we are forced to discuss this is fantastical and ridiculous. However, the Trump administration appears to be serious.

Such an extremely disastrous decision as invading Canada would lead to the Second American Civil War. Border states from Michigan to Maine would immediately refuse to comply with all orders from Washington. The National Guard units of these states could not be federalized in the face of what each governor would deem an unlawful order. This would set up clashes with guard units, police, and SWAT teams that may personally see themselves as loyal to Donald Trump. These two competing loyalties, one to Trump and one to the Constitution would exacerbate tensions on such a vast scale that disobeying orders would place the Armed Forces in a state of mutiny.

Additionally, some loyalist Trump units may take their arms and engage their fellow service members. The chain of events stemming from this is too horrific to fathom. But Team Trump is blathering on and on with a complete and total disregard for the consequences that could destroy the unity and fabric of this nation.