Mdthbs

Looking on Wikipedia, we learn that:

It is generally agreed that the commander-in-chief role gives the
President power to repel attacks against the United States[3][4] and
makes the President responsible for leading the armed forces. The
President has the right to sign or veto congressional acts, such as a
declaration of war, and Congress may override any such presidential
veto. Additionally, when the president's actions (or inactions)
provide "Aid and Comfort" to enemies or levy war against the United
States, then Congress has the power to impeach and remove (convict)
the president for treason. For actions short of treason, they can
remove the president for "Bribery, or other high Crimes and
Misdemeanors", the definition of which the Supreme Court has left up
to Congress. Therefore, the war power was intentionally split between
Congress and the Executive to prevent unilateral executive action that
is contrary to the wishes of Congress.

But the question is if the President can make direct orders without consulting any of his general through a phone call. For example, can the President order the U.S. military to launch a bomb against Canada without any reason? It is not explicitly stated if he can do so, but it seems that he can, but risk being impeached for such random and foolish decisions.

US soldiers swear to follow orders which are legal according to the Uniform Code of Military Justice, and are explicitly disallowed from following orders which are unlawful. Any Presidential power to use troops has to come from an Act of Congress per Article 1, Section 8, Clause 11 of the US Constitution

[The Congress shall have the Power] To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water

Congress has already granted the President broad power to use troops offensively without prior consent through the War Powers Resolution, but has also ratified membership in the NATO, which Canada is also a member of and almost certainly includes a provision against members attacking each other. Ratified treaties have equivalent power to an Act of Congress, so an officer would probably be justified in refusing to follow an order to attack Canada unprovoked without Congressional approval.

The Constitutionality of the War Powers Resolution is disputed by scholars, especially the effective blanket approval for military action without specific authorization from Congress. However, President is not legally required to consult with anyone prior to ordering a military action in general, and the legality of such an action would be determined after it was ordered - put differently, the President doesn't have to prove an order is legal before giving it, it's up to the military to refuse to follow it if it is unlawful.

Yes and no.

The War Powers Resolution (sometimes known as the War Powers Act) is supposed to limit it.

The War Powers Resolution requires the President to notify Congress within 48 hours of committing armed forces to military action and forbids armed forces from remaining for more than 60 days, with a further 30-day withdrawal period, without a Congressional authorization for use of military force (AUMF) or a declaration of war by the United States. The resolution was passed by two-thirds of each of the House and Senate, overriding the veto of the bill from President Nixon.

In theory, it limits the unilateral war actions of the President beyond 60 days. In practice, it has not really changed anything. Every President has violated the WPA since its passage, with all of them (both parties) believing it too be unconstitutional. Even Obama, who endlessly praised it

President Obama, in defending the legitimacy of the Libyan operation, hasn’t actually made that argument. On Wednesday, he submitted a report to Congress arguing that his administration isn’t in violation of the act at all, despite the fact that the 60-day deadline for congressional approval of Libya operations came and went in May.

President Obama is far from alone in finding creative ways around the War Powers Act. As the New York Times has noted, the Clinton administration continued the bombing campaign in Kosovo past the 60-day deadline, arguing that Congress had implicitly approved the mission when it approved funding for it. (The Act specifically says that funding doesn’t constitute authorization, the Times notes. And Obama wouldn’t be able to use that reasoning anyway—the administration is using existing funds for the Libya mission.)

Does that mean the President is not constrained by Congress at all? Hardly. As the House notes, they still hold the power of the purse

Congress—and in particular, the House of Representatives—is invested with the “power of the purse,” the ability to tax and spend public money for the national government.

Let's say a President tried to start a war without Congress. They would quickly find out the military did not have funding to run the war for very long. As a result, the military would be hamstrung in operating without funds. It would not bode well for anyone to be squabbling politically while troops were deployed to a military theater, but at that point, the war would become a direct political question. If people felt the war should not be pursued, they could elect people who would refuse to fund it. If people felt the President was right, they could vote people in who would fund it.

Let's finish this by talking about this hypothetical bombing of Canada. Without a good reason, the President would quickly find themselves on the losing end of public opinion (neither party would tolerate a direct act of aggression), and possibly facing impeachment.