Complex sentence: In the fifteen years since the F-104s had been purchased the advancements in engines, aerodynamics and especially mission electronics allowed for these roles to be combined in multi-role aircraft that were in the process of being introduced by a number of air forces around the world.
Split into simpler sentences: In the fifteen years since the F-104s had been purchased the advancements in engines, aerodynamics and especially mission electronics allowed for these roles to be combined in multi-role aircraft. A number of such designs were in the process of being introduced by air forces around the world.