Tens of thousands of Ukrainians now live under Russian control in their own country. Civilians in towns and villages from Kherson in the south to Izyum in the east are grappling with crackdowns and, evidence suggests, horrifying war crimes perpetrated by Russian soldiers.

Ukrainians living in Russian-controlled areas are also doing something else: resisting.

Ukraines military is leading the countrys effort to vanquish Russian President Vladimir Putins forces, which invaded in late February. The U.S. and allies from Europe to the Pacific are funneling arms and sophisticated equipment to these troops.

But Ukrainian civilians are stepping up, too. With little support and training, ordinary people in war-torn, occupied areas are undermining Russia, supporting their countrys counter-offensives, and fueling unity among Ukrainians and international concern for their nation.

Theres no way to over-exaggerate the extreme bravery that these ordinary citizens are demonstrating, said Olga Onuch, an associate professor at the University of Manchester.

The civilian resistance to Russia is using time-honored tactics, like daily demonstrations, and foiling occupiers plans to consolidate their rule by refusing to cooperate. The resistance is also developing covert, informal efforts to ease humanitarian suffering and hurt Russias military capabilities by gathering intelligence and training citizens in small-scale guerilla tactics.

The choices of everyday people can have huge implications for a campaign like the Russian mission, which depends on controlling strategic population centers and challenging a national identity, military experts say. Russia wants the world to believe Ukrainians want its incursion; their resistance proves otherwise.

Some of the most important flashpoints in the conflict in the weeks ahead are connected to occupied towns. Moscow is now focusing on eastern Ukraine, according to national security analysts and Russian leaders. Russias forces seek to hold Izyum, a city they captured on Friday that was once home to 40,000 people, and to conquer the nearby city of Slovyansk. If the Ukrainians can thwart that plan, they might quash Russias hope of cutting off Ukrainian troops in the countrys eastern regions from reinforcements and supplies.

And in the south, Ukrainians are defying Russia in towns like Melitopol, Enerhodar and Kherson areas that are integral to Russias attempt to limit Ukraines access to its coasts, seize the Ukrainian ports of Mykolaiv and Odesa and establish a land corridor between Russian territory and Crimea, the Ukrainian peninsula that Russia took over in 2014.

While Ukraines armed forces are proving themselves capable, no resistance can succeed without an effective auxiliary [of citizens in occupied areas] and underground. The civil element will be essential in the current conflict, Walter Haynes, a U.S. Army major, recently wrote in an essay for the blog War on the Rocks.