Treaties are only a part of Foreign policy, an important but nevertheless small part as well. It should be obvious that most of the Foreign Policy activity by the POTUS and the State department and roving congressional committees is unrelated to a treaty.

Sure. My point was that, even in foreign policy, Congress has some say. They have to ratify something for it to become law, for example. And once they do, it is law (that even future POTUS's can't just ignore). They also control the purse strings, so -just as an example- Ukraine aid is dependent on Congress. It's a foreign affairs matter, certainly. And POTUS can promise lots of tanks and Apaches. But Congress ultimately says whether or not we spend that money. No money, no toys for Ukraine. Pretty sure any trade deals have to get Congressional approval. Etc.

Congress has been shifting a lot of their responsibilities on the Executive for decades. They don't want to take a stand they can be held accountable for later. But, if it's working correctly, the POTUS shouldn't have that much power. Our Founders did not want another king. POTUS has veto power, and can use that to influence the course of America. But Congress spends the money, ratifies treaties and agreements, declares war, etc. At least, supposedly.