It’s like any international treaty, you agree to it because there’s something in it for you in return for signing it.
In this case, there’s a straightforward benefit to it in that it could be used to prosecute crimes against the US and US citizens, and soft benefits e.g. of the US being seen a a paragon of lawfulness and trust. There’s likely more, these are just what I could think of immediately.
If the US wants to prosecute someone who is residing outside it's jurisdiction, we do a rendition. Not all countries have that option.
Another benefit is discouraging our government from committing war crimes.
that only works if the ICC has any jurisdiction any power
That's true, but the member states agree to offer that jurisdiction. For instance, joining the treaty would allow us to arrest suspected war criminals, or at least, discourage them from traipsing around on our territory.
A lot of people toss around “war criminal” pretty loosely. And regardless of whether you think say, George W Bush was a WC (I don’t), it would be awkward for any country to have to hand over its elected leader, which could even be a popular one with broad support, to be prosecuted at The Hague, because Europeans disagreed with a military decision. Joining the ICC would mean the US would need to do just that.
> it would be awkward for any country to have to hand over its elected leader,
Since the leader (elected or not) generally controls the executive apparatus (that’s what makes them the “leader”), its unlikely that this would actually occur, irrespective of the nominal obligation. It is more likely that they would be turned over after being removed as leader (a process which an ICC warrant may or may not accelerate, depending on the response of other institutions, the populace, etc.)
At least, that is what has generally happened with the top leaders that have ended up at the ICC (or its predecessor ad hoc tribunals, back to Slobodan Milosevic, the first sitting head of state to be charged with war crimes by an international tribunal—the ICTY—who wasn't turned over by Yugoslav authorities until after he resigned.)
> because Europeans disagreed with a military decision
The majority of State Parties to the Rome Statute are non-European, as are a majority of the current judges of the court, as is the current acting chief prosecutor, so it is really weird to describe ICC involvement this way.
I suppose it would be like the governor of a state getting prosecuted for Federal crimes.
Would GWB have acted differently, had he been forced to consider the legality of his actions under a broader framework? Would we have preferred for presidents to have even broader immunity from prosecution?
The ICC demonstrably has some of each of those things, though less than proponents would like in both cases.