Why it matters: Ukrainian troops bombarded Russian ammunition depots, command and control centers and river crossings. The scale of the operation is not yet clear, but Ukrainian officials claimed they breached Russia’s “first line of defense” outside Kherson.

Kherson (pre-war pop. 300,000) was captured in the first days of the invasion by Russian forces advancing north from occupied Crimea. It remains the only provincial capital to fall in six months of fighting. Ukrainian officials have been heralding an attack on Kherson for weeks, and Russia has roughly doubled its force in the region in preparation, according to Poland-based Rochan Consulting. National Security Council spokesman John Kirby noted that to do so, Russia has had to “deplete some units in the East” — where a major Russian offensive has largely stalled — and faces “manpower challenges.”

Between the lines: So does Ukraine. Both sides have suffered heavy casualties in four months of bitter war in the eastern Donbass region and are scrambling to train (and in Russia’s case, recruit) reserve forces.

While Ukraine has successfully deployed longer-range US-supplied missile systems to conduct strikes well behind the front lines, some analysts doubt that Ukraine has assembled the necessary manpower or material to launch a major attack on a well-protected city. Others have noted that Kyiv may be politically motivated to move now, to boost morale and reassure foreign backers by showing momentum, even if the military outlook is bleak.

The big picture: Without the numerical advantages typically required for a successful offensive, Ukraine can focus on cutting off Russian forces in Kherson, rather than attempting to drive them out.

Ukrainian President Volodymyr Zelensky gave no hints in a speech on Monday night, insisting that Russian forces would be expelled from Ukraine, but added: “You won’t hear details from anyone really responsible. Because this is war.” Even if Ukraine proceeds cautiously, the offensive likely marks the beginning of a third phase of the war, following Russia’s initial three-pronged offensive and the fierce fighting in Donbass.

What to watch: Another twist could happen within days if Russia announces a fake referendum in Kherson as a pretext for annexing the territory to Russia, as Kirby has warned could happen.

Russia has already installed a puppet government, restricted the Internet, enforced the use of the ruble, and started issuing Russian passports. The latest: Russian investigators confirmed today that a former Ukrainian lawmaker who defected to join this puppet government had been murdered in his home.

Meanwhile… The launch of the attack comes as International Atomic Energy Agency (IAEA) inspectors head to Zaporizhia in southern Ukraine for an urgent examination of Europe’s largest nuclear power plant, which has been damaged by bombing.