I think a lot of people would agree that by far the most serious threat to American constitutional democracy, particularly its guarantee of individual rights, is the ongoing expansion of Presidential power. Generally that position is held by most Democrats when a Republican is President and held by most Republicans when a Democrat is President, so they're probably both right.
There's no dispute that the Executive Branch has the sole authority to wage war, with the President as Commander-in-Chief. It couldn't be any other way. The big questions are over who sets policy for waging war, and where's the line between policy and execution? The Constitution clearly establishes Congress, not the President, as the policy-making branch of the US federal government. Personally, I think it's also clear that this principle doesn't go out the window in wartime. That's why Congress, not the President, has the sole authority to declare war.
Congress also has the closely related authorities to raise an army and navy, to collect taxes to pay for them, to allocate those funds, and (for the Senate) to ratify all foreign treaties.
For the most part, the US Constitution makes brilliant use of the natural human desire for power. That's the whole idea behind the carefully crafted set of "checks and balances" that every civics class talks so much about.
But it has an Achilles heel that will probably be its (and our) undoing. Presidential power always expands greatly in wartime and shrinks in peacetime. The bigger the war, the greater the expansion. World War 2 was the extreme case - so far - and it coincides with the start of an enormous expansion of Presidential authority that still continues. (The first and only President to serve more than 2 terms was in office for nearly all of WW2, leaving only when he died.) This strongly tempts every President to start or expand a war on a pretense merely to expand his own power. His ability to do this is now greatly enhanced by his extremely tight control over the gathering and dissemination of the information most relevant to the decision to go to war. Even though Congress still makes the nominal decision, it is almost preordained by the information given to them.
I'd like to think that there could never be a more pure example of the US going to war solely to expand the power of the President than George W. Bush's invasion of Iraq. But that would be wishful thinking. This is easily the most dangerous structural weakness in our entire government and I fully expect it will be our eventual undoing.