Complex sentence: However, the Grenzers were first and foremost skirmishing troops and were thought to not perform as well in the role of line infantry as the regular regiments, this caused many in the Austrian military command to not hold them in much esteem.
Split into simpler sentences: However, the Grenzers were first and foremost skirmishing troops and were thought not to perform as well in the role of line infantry as the regular regiments. As such, many in the Austrian military command to not hold them in much esteem.