Complex sentence: On the right flank the legions XI and VIII had combined their efforts in containing the Viromandui and pushed them back to the river bank, however they were unable to drive them across the river or defeat them.
Split into simpler sentences: On the right flank, the XI and VIII legions had combined their efforts in containing the Viromandui and pushed them back to the river bank. However, they were unable to drive them across the river or defeat them.