Complex sentence: Quatre Bras was the rendezvous of the 2nd Brigade (Goedecke's), and the 3rd Battalion of the 2nd Regiment of Nassau, which was cantoned in its immediate vicinity, had already, without waiting for the receipt of superior orders, assembled at that point.
Split into simpler sentences: Quatre Bras was the rendezvous of the 2nd Brigade (Goedecke's), and the 3rd Battalion of the 2nd Regiment of Nassau. The 2nd Regiment was cantoned in its immediate vicinity of Quatre Bras, and without waiting for the receipt of superior orders, assembled at that point.