 Complete the following phrase:  The current RC-135 fleet is the latest iteration of modifications to this pool of aircraft dating back to the early 1960s. Initially employed by Strategic Air Command for reconnaissance, the RC-135 fleet has participated in every armed conflict involving U.S. forces during its tenure. RC-135s supported operations in Vietnam War, the Mediterranean for Operation El Dorado Canyon, Grenada for Operation Urgent Fury, Panama for Operation Just Cause, the Balkans for Operations 