Complex sentence: At the outbreak of war, this force was renamed "Armoured Division (Egypt)" and became the 7th Armoured Division, nicknamed the "Desert Rats", the division was the principal force defending the Egyptian border with Libya at the start of the war.
Split into simpler sentences: At the outbreak of war, this force was renamed "Armoured Division (Egypt)" and ultimately became the 7th Armoured Division. The 7th Armoured, nicknamed the "Desert Rats", served as the principal force defending the Egyptian border with Libya at the start of the war.