Complex sentence: On 9 February 1945 a force of Allied Bristol Beaufighter aircraft suffered heavy casualties during an attack on German destroyer "Z33" and escorting vessels which was subsequently labelled "Black Friday" by the surviving Allied aircrew.
Split into simpler sentences: On 9 February 1945 a force of Allied Bristol Beaufighter aircraft suffered heavy casualties during an attack on German destroyer "Z33" and escorting vessels. This operation was labelled "Black Friday" by the surviving Allied aircrew.