The Donbass did not have such a history nor such a clear majority in favor of secession. So, in that case Russia offered limited and covert protection for the Donbass against violence from the central government while advocating that both sides sit down and negotiate.
The Catalonian independence movement is the direct result of the EU and, in some ways, it is surprising that it took this long for a major independence movement to arise. The nation state in Europe has had the problem that the EU weakens national identity because some of the sovereignty has been transferred to the EU. Because of this, by comparision, regional indentities have become more important than national identities. This is a phenomenon not unique to Spain. Certainly, U.K., Germany, Italy are all good examples of EU nations with identifiable regional identities.
Exactly how it will play out is impossible to say with precision. However, it is possible that Texas will reach a point similar to Catalonia where they do not see that the submission to the majority on a series of issues is sufficiently compensated by the benefits of the union. After all, Texas is larger than many significant countries and has a very nice coastal region. It is a viable nation state.
If Texas secedes, the Republican Party will become a permanent minority and strongly red states will likely follow suit and sue for membership in the new Republic of Texas. It is quite possible that Houston's Harris County will not secede as well as a Democratic swath of counties along the Rio Grande. There are probably several other states whose majorities would vote for independence where urban counties would not secede with them.
Given the political polarization that Pew Research has found to have emerged over the past decade, public opinion is actually ripe for a secessionist movement. One way that it might begin is with County referenda on whether the County favors membership in a Conservative nation. The referendum might be phrased like this - 'If a portion of the U.S. became a separate, more Conservative, nation would you favor XYZ County becoming part of it?'
Such referenda would have no legal force and would likely be declared illegal by Federal courts. However, they probably would not be taken any farther than the press reporting that XYZ county has voted to join a hypothetical, more conservative, nation if it were to come into existance. Neither the Federal government nor the county would attempt to move any further.
Unlike the first U.S. secession, where northern leaders were strongly in favor of forcing the South to remain part of the Union, today, many Democratic thought leaders are moving toward the view that it would be better for the rest of the U.S. if Texas seceded. They see a long string of legislative initiatives that are being thwarted by red state majorities in Congress such as gun control, the death penalty, climate change, etc. Also, a number of issues currently settled or mostly settled, such as ACA, abortion, gay marriage ,etc. are being put under pressure with every election. The successful secession of just a handful of strongly red states would likely turn the U.S. permanently blue and allow those states that remain to move toward a European style social democracy.
There is a serious question whether the strongly liberal northeast and the more laid back liberal Pacific coast, if separated by a mid continent conservative nation could continue on as a single nation. The rest of EUNA (roughly equivalent to the NATO nations) would probably see the eventuality of three daughter nations as beneficial. Far too often, due to its sheer size, the U.S. has behaved like a playground bully. The three nations would be closer in population and GDP to other EUNA nations and would have foreign policies that would differ significantly from one another.