Broker a ceasefire, to start, as that ends the immediate loss of life. Begin negotiations for a permanent peace settlement. Establish terms for repatriating land to displaced refugees and compensation for death and destruction of property.
Then hold elections in Ukraine to establish a new postwar government that can continue the peace process.
letting Russia seize control of the entire nation
The only way to prevent Russia from furthering its march into Ukraine is to establish a ceasefire.
Ukrainians had that chance in 2022 and they choose counteroffensive. It failed and they lost territory.
In 2023, they tried again and failed, and lost more territory.
Now Kyiv is under regular bombardment from fortified Russian positions in the North and the entire Donetsk line is under unsustainable pressure.
Either Europe enters the war to bail out Ukraine, Russia accepts a ceasefire and begins a peace settlement, or you’re looking at another big line collapse and retreat, with commensurate territory loss all through the East and South.