Because if they don't Slavyansk and Kramotorsk -- which are the largest cities in DNR under Uke control -- become immediate target for the Russians. And their liberation will be the end of the Donbass battle.
Now, how successful they can the Ukes be with this new line is not clear, and it is not really clear where the line will be drawn.
Below is the current map.. Bahmut/Artemovsk is just outside in the low left corner. Two large cities in the reddish (Russian) part of the map are Severodonetsk and Lysychansk.
The large city in the greenish corner of the map is Seversk. Are the Ukes going to try to defend it or not? -- this is the immediate question. Because if not, Bahmut to the SW and Slavyansk/Kramatorsk are next.
