russian advance. they could be tested with about 100,000 russian troops, tanks and artillery massed along three sides of the ukrainian border and by pro-russian militias already inside ukraine this is the most dangerous flash point. ukrainian troops occupy these trenches 24/7 and russian soldiers are only 50 yards away according to the ukrainian soldiers here those russian backed troops fire on them almost every day. and it wouldn't take much for an escalation here to trigger a much wider war. >> when some side start that attacking, it's -- it have casualties no matter what they do. >> reporter: lieutenant evan showed us his front line position, a rubber factory devastated by an eight-year war with pro-russian separatists that now has the potential to trigger the biggest conflict in europe since world war ii