JUDY WOODRUFF:

Tonight, we begin a weeklong series from Eastern Europe that we're calling Fault Lines.

On Friday, NATO will announce the largest military buildup in Europe since the Cold War. Tensions between the West and Russia have reached the highest level since the fall of the Soviet Union. This week, we will examine the causes of that tension.

Tonight, we begin with Europe's only active front line, in Eastern Ukraine. For two years, fighters for the self-proclaimed Donetsk People's Republic, with the backing of Russia, have fought the Ukrainian government to gain autonomy. The West, including the U.S., is backing Ukraine's government; 10,000 people have died.

With the help of the Pulitzer Center on Crisis Reporting, special correspondent Nick Schifrin and producer Zach Fannin traveled to Donetsk, and discovered that what is supposed to be a cease-fire is anything but.