I understand what you are saying, but usually (and Iran is a good example) the trade agreements under treaties with such countries also supply spare parts. In the event of conflict injurious to our countries well being, the supply of spare parts ceases and it isn't long until the machines grind to a halt. Using Iran as an example, one site says the following: "By 1987, however, the air force faced an acute shortage of spare parts and replacement equipment. Perhaps 35 of the 190 Phantoms were serviceable in 1986. One F-4 had been shot down by Saudi F-15s, and two pilots had defected to Iraq with their F-4s in 1984. The number of F-5s dwindled from 166 to perhaps 45, and the F-14 Tomcats from 77 to perhaps 10. The latter were hardest hit because maintenance posed special difficulties after the United States embargo on military sales."
So, thier only recourse is to try an rearm with suppliers from North Korea, China, and lately, it seems, Russia. The crews all have to be re-trained in the use of new machines and, in most cases, entire weapons systems, which for the most part are sub-par to those supplied by the U.S. and the U.K. (and other European countries)...