Just start with, "I need to talk to you about our relationship," then tell her what you just told us. Relocating is a deal breaker for you, and that's a reasonable deal breaker. Always use "I" statements, and never "You" statements.

There's never a good way to break up with someone, but doing it in person and being straight forward and honest is the best path to use. And you never know when a clearly communicated counter argument might actually change someone's mind

BUT do not go into this thinking she's going to change her mind. Expect her to stick to her guns and the relationship to be over.

"That doesn't work for me."

Also,

"You creating and mandating a plan for our future that doesn't involve discussing my wants and needs shows that you don't value them"

I don't have any idea what the rest of your relationship is actually like. From the little here, sounds like you're ready to move on, and so is she. If she's giving you that ultimatum from another country she may also be hoping you do the hard part and break up with her, relieving her of whatever feelings of guilt that might be associated. Remember though... No guilt is necessary from either of you to move on. You want different things from the time that you have, and that's ok.

You're both at an age where, especially if you want to get married and have kids, neither of you have time to spare on misaligned relationships. Having children and living long-term in a foreign country are two of only a handful of dealbreakers when it comes to relationships, no matter how perfect you otherwise are for each other. Which sucks, and is really painful to realize, but at least you both seem to know exactly what you want, and it seems like you at least have realized what you want and what she wants are incompatible.

You know her best and know what to say to her best. But generally speaking, it sounds like you need to psych yourself up for a serious conversation where compromise is not feasible and a breakup is the ultimate outcome. It's going to be a painful conversation, and the temptation on both sides to budge and try to come up with a way to make it work will be strong. But if you stay together someone is going to be miserable with the situation; they may deal with it for a few years, but it'll all come crashing down eventually. Do you want to be in your 40s realizing that your marriage (with kids!) isn't working, and now you have to deal with international shared custody?

Personally I think it’s always best to accept an ultimatum (as in, bye Felicia) part of having a grown-up relationship is being able to communicate, discuss and compromise with your partner. You have to tell her that you’re not interested in moving back, It’ll be easier to do it sooner rather than later. Not living together makes this a lot easier

Ultimatums are almost always red flags in my book.

Just tell her you don't want to relocate, and given that's what she wants to do, it's best to amicably go your separate ways. That's hobestly such a big deal, that it should be enough of an explanation.