Complex sentence: Its importance increased throughout the second half of 1943 and into the spring of 1944, until it became the largest BCRA network in the Vichy zone, employing about 2500 sources, contacts, couriers and analysts.
Split into simpler sentences: Its importance increased throughout the second half of 1943 and into the spring of 1944. It eventually became the largest BCRA network in the Vichy zone, employing about 2500 sources, contacts, couriers and analysts.