Well, sounds like you're stuck. THe original terms of the agreement have changed (length), and he's free not to agree with a new agreement.
On the other hand, unilateral decisions are pretty much a death knell for a relationship, as it brings about Independent Behavior (IB), a sure-fire lovebuster. On the other hand, your H is at least in spirit, reneging on certainly the spirit of the agreement, if not the letter.
Do you really think a 4 and 8 your old are capable of understanding what you're asking of them? Let alone the stress of separation and divorce and the associated abandonment, now even when M and D are together, people still leave of their own volition?
Is there no other thing you can do in the military that keeps you closer?
Ultimately, I don't think there's a right answer anywhere. Somebody's gonna get hurt. perhaps irreparably. The path of least damage seems to be for you not to go, as it keeps the bonds with the kids strong at those young years, and allows you and your H to work on your budding romance, assuming he can get over his insecurities. The only person that really loses out on their desire (and it's a want, not a need) is you.
On the other hand, if you go, 3 people suffer, and one gets what they want. Do you think that when you get back you will be a better wife and mother because of what you're doing? That somehow your children and H will respect you more and such?
Very few people on their deathbed wish they'd worked more. It's almost always family that is more important.
I don't know what your belief system is, but career on almost any value system usually places below family.
My final guess is that if you go, it will do long term damage to your relationship that will be practically impossible to recover from. And whatever fleeting joy you receive from your tour will be instantly lost when you come home and face over the years the long term damage to your family.
Ball's in your court. Be curious what you decide, an dperhaps when you get back you can report back and let us know how it's going.