Control of Luhansk, and the wider Donbas region, could hinge on the battle for Sievierodonetsk.

The battle for Sievierodonetsk, which could fall to the Russians within days, is about far more than one city. Its capture would give Russia a key victory in its drive to seize the entire Donbas region of eastern Ukraine.

Donbas, which comprises the territories of Luhansk and Donetsk, is a prize for President Vladimir V. Putin of Russia. After failing to swiftly topple Ukraine’s government in Kyiv, Mr. Putin refocused his military campaign on the Donbas, which makes up about 9 percent of Ukraine’s land, but holds significance for its industry, location and potential as a bargaining chip for Moscow.

The Donbas borders Russia and runs from outside Mariupol in the south to the northern border near Kharkiv, Ukraine’s second-largest city. Home to coal mines and steel, an estimated 6.2 million people lived in the region before Russia’s invasion, according to the most recent census data.

Kremlin-backed separatists have held territory in the Donbas since 2014, after Russia annexed Crimea, a move that Ukraine and its Western allies have termed illegal. The proxy forces seized more than a third of the Donbas at the time and proclaimed the formation of two breakaway republics; they have been waging a civil war against Ukraine’s government ever since.

More than 14,000 people have died in the fighting since it began. But the battle was effectively at a stalemate — albeit one with sporadic and deadly shelling along the roughly 250-mile front line known as the line of contact — until Russia invaded Ukraine on Feb. 24.

Now Russian and forces and their separatist allies control an estimated 80 to 90 percent of the Donbas, according to Ukrainian officials.

Capturing Sievierodonetsk and the neighboring city of Lysychansk would give Russia full control of Luhansk, and position its forces to advance farther west toward the city of Kramatorsk in neighboring Donetsk province, one of the last major cities still under Ukrainian control there. If Kramatorsk falls, Mr. Putin’s forces would in effect control the whole Donbas region.

That’s critical from a supply standpoint as well as a symbolic one. Mr. Putin could claim a tangible military victory and use the territory as leverage in any future peace negotiations with Ukraine. Holding the Donbas would also expand Moscow’s “land bridge” connecting Russian territory to Crimea.

But a full capture of Donbas is far from assured. Pushing westward from Sievierodonetsk could strain Russian supply lines, which proved vulnerable in its early failures to seize Kyiv and other parts of northern Ukraine. And the Russian military’s tactics of scorched-earth bombardment mean that any territory gained will likely bear widespread destruction and need extensive rebuilding.