Complex sentence: A catastrophe for the Allies in Egypt might in turn lead to destabilisation of Britain's control of Middle Eastern oil supplies or their capture by the Axis but this depended upon Axis forces in North Africa receiving adequate supplies from Italy.
Split into simpler sentences: A catastrophe for the Allies in Egypt might in turn lead to destabilisation of Britain's control of Middle Eastern oil supplies, or even worse, to the Axis gaining control of them. This scenario depended upon Axis forces in North Africa receiving adequate supplies from Italy.