Russia's annexation plan for Ukraine

 ISW:

Key Takeaway: Russian President Vladimir Putin likely intends to annex occupied southern and eastern Ukraine directly into the Russian Federation in the coming months. He will likely then state, directly or obliquely, that Russian doctrine permitting the use of nuclear weapons to defend Russian territory applies to those newly annexed territories. Such actions would threaten Ukraine and its partners with nuclear attack if Ukrainian counteroffensives to liberate Russian-occupied territory continue. Putin may believe that the threat or use of nuclear weapons would restore Russian deterrence after his disastrous invasion shattered Russia's conventional deterrent capabilities.

Putin’s timeline for annexation is likely contingent on the extent to which he understands the degraded state of the Russian military in Ukraine. The Russian military has not yet achieved Putin’s stated territorial objectives of securing all of Donetsk and Luhansk oblasts and is unlikely to do so. If Putin understands his military weakness, he will likely rush annexation and introduce the nuclear deterrent quickly in an attempt to retain control of the Ukrainian territory that Russia currently occupies. If Putin believes that Russian forces are capable of additional advances, he will likely delay the annexation in hopes of covering more territory with it. In that case, his poor leadership and Ukrainian counteroffensives could drive the Russian military toward a state of collapse. Putin could also attempt to maintain Russian attacks while mobilizing additional forces. He might delay announcing annexation for far longer in this case, waiting until reinforcements could arrive to gain more territory to annex.

Ukraine and its Western partners likely have a narrow window of opportunity to support a Ukrainian counteroffensive into occupied Ukrainian territory before the Kremlin annexes that territory. Ukraine and the West must also develop a coherent plan for responding to any annexation and to the threat of nuclear attack that might follow it. The political and ethical consequences of a longstanding Russian occupation of southeastern Ukraine would be devastating to the long-term viability of the Ukrainian state. Vital Ukrainian and Western national interests require urgent Western support for an immediate Ukrainian counteroffensive.
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There is much more.

I do not think the US and much of the western world would recognize such an annexation of that part of Ukraine.  The sanctions would stay in place and those places would generate little in the way of trade.  It would be seen as an illegitimate attempt to impose rule over the area.