Complex sentence: It was the first big operation by the Eighth Army since the Second Battle of El Alamein months previous; the first attack, Operation Pugilist ran into difficulties, when after a bridgehead was established, a breakout attempt failed against determined Axis counter-attacks.
Split into simpler sentences: It was the first big operation by the Eighth Army since the Second Battle of El Alamein months previous. The first attack (Operation "Pugilist") ran into difficulties: a bridgehead was established, but the breakout attempt failed against determined Axis counter-attacks.