Okay guys I think you've guessed what happened but I might as well post these two screenshots for good measure from a 7.1.8 standalone Op Star scen I've just finished, medium difficulty.
PIC 1- this is turn 17, I've held Kharkov and Merefa (big red arrows) for the required 15 turns but the game still hasn't ended and I still haven't got a beautiful green tick(sniffle).
That's because I haven't yet killed 15 tanks (red circle).
PIC 2- sulking and pouting like crazy, I go back into the game (it's still turn 17) and kill the 15th tank and THAT'S A BINGO! and the game instantly ends on turn 17 and this Victory screen comes up which put a smile on my face bigger than a wave on a slop bucket..
Conclusion- we have to achieve ALL Prims and BOTH Secs to get a victory, I wish the scen designer had told us that, grrr..