Complex sentence: On the night of November 15,1858, seven battalions of the imperial force were sent to attack the Taiping army at Golden Ox Town under the cover of darkness, but unbeknownst to the imperial troops, the Taiping army had already laid an ambush for them.
Split into simpler sentences: On the night of November 15 seven battalions of the imperial army were sent to attack the Taiping army at Golden Ox Town under the cover of darkness. Unbeknownst to the imperial troops, however, the Taiping army had already laid an ambush for them.