Ukraine has always been viewed as a backwater by Russians. They think the people are hicks and their language is hick talk. Remember the languages share a lot of words in common. The Czars deliberately kicked Ukrainians out of their cities and resettled the cities such as Kyiv with Russians which reinforced this stereotype. At the same time, English capitalists were showing up hoping to start factories, and the Czars decided that the area now know as the Donbass would be a good spot. The Czars made sure that the new cities springing up around the new factories were thoroughly Russian even though the countryside spoke a language more like Ukrainian at the time.
Nonetheless, when the Soviet revolution started, Ukraine fought for independence against all comers - Austria, Germany, Poland, the Russian White army, and the Red Army all tried to re-colonize Ukraine as it fought for independence. The Red Army won, unfortunately. The Soviets kept up the Czarist pattern of resettling Russians or loyal non-Russians in strategic areas far from Russia, particularly along the Dniester River and Crimea (they did this in other countries as well, all of the -stans basically). This wasn't enough though and the Soviets wanted to make Ukraine confirm even more to Russia, so they moved the border to include areas that were always Russian.
Since independence, Ukraine has indeed made efforts to assimilate these areas and force the kids to learn Ukrainian in school instead of Russian. It generally hasn't gone well. If that was the only problem we might say, sure, let Putin reclaim the land that Kruschev gave away. But that is not the only problem. Latvia has the same problem, with Russians who have always been Russian in their borders.. And Latvia is in NATO. It would set a bad example, to say the least.

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