Complex sentence: The attacks were initially successful in pushing weak units of the Austro-Hungarian First Army out of the mountains, but the Austro-Hungarians sent four divisions to reinforce their lines, and by the middle of September, the Romanian offensive was halted.
Split into simpler sentences: The attacks were initially successful in pushing weak units of the Austro-Hungarian First Army out of the mountains. But the Austro-Hungarians sent four divisions to reinforce their lines, and by the middle of September, the Romanian offensive was halted.