Complex sentence: Lieutenant General John Burgoyne's 8,000-man army occupied high ground above the fort, and nearly surrounded theunder-strength force of 3,000 under the command of General Arthur St. Clair, to withdraw from Ticonderoga and the surrounding defences.
Split into simpler sentences: Lieutenant General John Burgoyne's 8,000-man army occupied high ground above the fort, and nearly surrounded the defences. These movements precipitated the occupying Continental Army, an under-strength force of 3,000 under the command of General Arthur St. Clair, to withdraw from Ticonderoga and the surrounding defences.