Armenia and Azerbaijan have declared martial law and mobilized their militaries, after heavy fighting broke out Sunday between the two nations in a disputed territory in the Caucasus Mountains. There are reports of at least 23 dead and over 100 wounded, including civilians, as both sides traded tank, artillery and drone fire in a region that’s internationally recognized as Azerbaijani territory but whose population is mostly ethnic Armenians. It’s the worst fighting in the area since a five-day war in 2016 left over 100 people dead.