That's a major part, but the key problem IMO is that the USA has shown multiple times that a change in president can be enough to shatter existing promises and expectations. Even if we got the most progressive, effective president in the history of our nation, anything they accomplished could be undone by a single change of admin.
That’s a major part, but the key problem IMO is that the USA has shown multiple times that a change in president leadership can be enough to shatter existing promises and expectations.
FTFY
The US is fighting to even pay it's own debts because of the political grabass going on in the Legislative branch and who knows now what the Judicial branch is going to throw up to change. The President is just the most visible change.
It's mostly just that one time. Usually foreign policy continues from where it left off much to the chagrin of the voting base who wants it all to change overnight. And this is because of the reason you stated, you don't want to break promises to other countries or everyone will thing the US is unreliable and that's bad for the US. Sure foreign policy can be turned and start slowly moving on a different path, but that has to happen over time.
Trump is the real outlier. He simply doesn't understand foreign policy, thinks of everything as a money deal. His incredibly corruption on foreign policy led to him being impeached the first time. Causing rifts within NATO. Legitimizing North Korea by having a summit. Gushing over how great various authoritarians are. Ended anything Obama did out of spite. Cancelled Obama's deal with Iran, which looked like a prelude to war because that's what it would be if the US had competent foreign policy.
In the first months of Trump's Presidency when he was putting out aggressive statements about certain countries (usually on Twitter) countries would put their militaries on alert. After awhile no one bothered. The US simply didn't have a leader anyone took seriously.
This is the reason for Biden's statement that "America is back." But unfortunately the US Congress is blocking funding that's needed for Ukraine, which would normally be a no-brainer in terms of foreign policy interests of the US. Usually something like that would go through with as a bipartisan thing with little to no negotiation necessary. Because it's in the best interests of the US to fund Ukraine's war effort against Russia, an adversary of the US. But it's blocked, because the GOP willing to sabotage the US in their effort to sabotage Biden.
So a Biden second term will mean the US isn't saying insane things anymore, but really can't be relied upon as it used to.
A Trump second term? Chaos. Not even sure the US can survive another Trump term, let alone be relied on for anything.