Complex sentence: Jelačić and his Croat army retreated towards Vienna for two reasons: first, he knew he would get some support from Vienna; second, he could not retreat, because militiamen and territorials were occupying South Transdanubia.
Split into simpler sentences: Jelačić and his Croat army retreated towards Vienna for two reasons. Secondly he couldn't turn back, because militiamen and territorials were occupying South Transdanubia.