Complex sentence: The political situation remained uncertain until May 13,1861 when Union troops occupied the state, restoring order and preventing a vote in favour of Southern secession, and by late summer Maryland was firmly in the hands of Union soldiers.
Split into simpler sentences: The political situation in Maryland remained uncertain until May 13,1861 when Union troops occupied the state, effectively preventing a vote in favour of Southern secession. By late summer Maryland was firmly in the hands of Union soldiers.