Well the Dominicans did offer to become a US state...
I once did a parody of this for a tape I made a friend for listening to while he was riding with his parents to visit relatives several states away. (This was in high school, I made a few of these.) As part of the plot, a poor, third world nation offers to sell *everything* they own to the U.S. - then just asks to bome a state even though it was on the other side of the world.
It included some other weird stuff as well - the President and Vice President resigning and appointing each other severn times, Congress keeps accepting, and so the guy's like 44th, 46, 48, 50th...president

And, the Soviets nuking the Chinese capital, with the President saying, "So that explains why I can't get ahold of the Chinese Premier."
All this is to say that there are those times it happens, but they tend to get out of control when people don't stop to think of how it could be done and the circumstances that revolved around it. On both sides - including those who call it "forced" when, as I say, the times the U.S. did buy land were sort of like going down to the used car lot and trying to talk the guy down. (Pictures wild, loud used car ad with, instead or cars, the owner sitting in an executive chair and saying "Nobody...NO-BODY...sells territory for less..."
Hmmm, would make an interesting ASB thread if someone did that in a country.