Complex sentence: Significant numbers of British Churchill and Matilda tanks along with US M3 Lee were shipped to the USSR after becoming obsolete on the African Front with the Churchills supplied by the arctic convoys seeing action around Leningrad while tanks shipped by the Persian route supplied the Caucasian Front.
Split into simpler sentences: Significant numbers of British Churchill and Matilda tanks along with US M3 Lee were shipped to the USSR after becoming obsolete on the African Front. The Churchills, supplied by the arctic convoys, saw action around Leningrad while tanks shipped by the Persian route supplied the Caucasian Front.