“Ukraine sent their tanks to Donetsk and Luhansk, in violation of the peace agreement they signed with Russia. That’s why Russia intervened.”
Do you mean the Minsk agreement that called for Russia to withdraw from eastern Ukraine, but it refused to do so? That one?
Donetsk and Luhansk are Ukrainian sovereign territories; it is not Russian territory. Fighting broke out because the Russian puppets refused to implement the Minsk agreement by conducting a vote that would have lead to autonomy within a united Ukraine. But more to the point, there would not have been any need for the Minsk agreements if Russian had not earlier invaded these Ukrainian provinces.