Complex sentence: The first Italo-German offensive had been an operational success but supply constraints made it impossible to advance further than the Egyptian border and the Axis forces were distracted by the siege of Tobruk, while the British could rebuild their strength in Egypt.
Split into simpler sentences: The first Italo-German offensive had been an operational success but supply constraints made it impossible to advance further than the Egyptian border. The Axis forces were distracted by the siege of Tobruk, while the British began to rebuild their strength in Egypt.