Canada

Paparo, who serves as U.S. commander for the Indo-Pacific region, visibly took issue with the question. He said: “I find that scenario to be absolutely and utterly preposterous.”

He added: “I follow the orders of the commander-in-chief, but that is a needlessly emotional question.”

Look at his left chest - a billboard of medals and citations. He didn't get those by giving the wrong answer, or by basing his statements or actions on his personal opinion. His 'preposterous' statement would be a statement of fact, not a statement of opinion. Here are some of the facts that are behind the factual 'preposterous' statement.

Any order that comes down the chain of command, keeps going down the chain of command. At every step of the way, the officers would have the option to follow the order by 'advancing to the rear'.

From Google AI: Presumption of Legality: Routine orders carry a legal presumption of lawfulness, and a soldier must obey normal commands. However, that presumption breaks down when an order directs an overt, clear violation of domestic or international law (such as targeting unresisting civilians).

Also: The defense of "just following orders" (historically known as the Nuremberg defense) is not legally valid in the US military. If a soldier carries out an order that a person of "ordinary sense" would know is illegal—such as executing a surrendered enemy combatant—the soldier can be criminally prosecuted alongside the officer who gave the command.

Also: The Supremacy Clause: Article VI of the Constitution states that treaties made by the United States are part of the "supreme law of the land."

Treaty Conflicts: If the U.S. has a formal treaty or defense pact with an ally, attacking that ally could violate U.S. treaty commitments, which hold the status of federal law.

Since both countries are members of NATO, any order to attack Canada from the Commander-in-Chief could easily be interpreted up and down the chain of command as an illegal order. This would put every soldier in the predicament of possibly being prosecuted for obeying such an order, right up and down the entire chain of command. It also presents the possibility of the POTUS being charged and convicted of the crime of issuing an illegal order. Thus, answering the question as stated is factually 'preposterous'. The POTUS could not, as Commander-in-Chief, be reasonably seen as being able to give a legal order to attack Canada.

But it is also very clear that he is indirectly accusing Trump of being needlessly emotional in even insinuating the possibility that Trump could give such an illegal order.