Complex sentence: On the outbreak of war, the regiment immediately transferred back to the UK, acting under 1st Motor Machine Gun Brigade in the summer of 1940, and then under 26th Armoured Brigade in 6th Armoured Division later in the year.
Split into simpler sentences: On the outbreak of war, the regiment immediately transferred back to the UK. It then came under 26th Armoured Brigade in 6th Armoured Division later in the year.