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