Complex sentence: In case of a collapse of the front between Dobro Pole and Tzena, the 1st Bulgarian and 11th German armies would either be annihilated or, in less favorable circumstances, perform an organised retreat to a new defensive line on river Crna.
Split into simpler sentences: In case of a collapse of the front between Dobro Pole and Tzena, the Bulgarian 1st Army and the German 11th Army. Armies would either be annihilated or, in less favorable circumstances, perform an organised retreat to a new defensive line on river Crna.