Vladimir Putin has formally annexed four occupied territories in Ukraine, the biggest land grab in Europe since World War II and one of the most egregious violations of international law since then.

It is a key moment in the war with major implications for what happens next.

Russia has annexed 15% of Ukraine’s territory, including several major cities -- but right now none of the areas Putin is seizing are under full Russian control and all are facing Ukrainian efforts to retake them.

The annexation will absorb the self-declared People’s Republics of Donetsk and Luhansk in the Donbas region, as well as parts of the southern Zaporizhzhia and Kherson regions that Russia occupies.

At a ceremony in the Kremlin today Putin signed “treaties of accession” with the Russian-installed leaders of the regions.

This is another no-going back moment for Putin. By making these territories part of Russia itself he has made negotiations even more difficult. He has locked himself into a long war and linked the survival of his regime to it.

He cannot give up the regions in negotiations -- in 2020, when he changed the constitution to let him stay in power beyond his term limits he also introduced a new clause that forbids Russian president’s from giving up any Russian land.

But perhaps even more importantly, he is likely to lose parts of these regions -- Ukraine is on the counteroffensive still in northeast Donbas and Kherson.

Most experts believe that for now Putin is very unlikely to use a nuclear weapon -- they see his threats as bluffs. But, they say the risk he might is growing and is now the most serious it has been.

For now, many experts believe Putin would prefer to use mobilized troops to try to stabilize Russia’s front lines in Ukraine and then try to outlast the West through the energy crisis this winter. But should Ukraine continue to advance and Russia’s position in the newly annexed regions starts to collapse, the risk he will use a nuclear weapon could grow.