Dutch Air Force F-16 fighters in 2018. The Netherlands has pledged 24 jets but says it will retain them until Ukraine is ready for them.Credit...Horacio Villalobos/Corbis, via Getty Images The jets are ready, and the flight instructors are waiting, at a new training center in Romania that was created to teach Ukraine’s pilots to fly the F-16 warplane. But there’s a catch: The Ukrainian pilots have