Maybe Crono didn't even know what was going on...Maybe he was just chillin in Truce or on (affectionatly called) Ashtear Island w/Lucca.
Ooooo...I like the sound of that (Ashtear Isle I mean).
Anyway, the point is, almost every1 agrees that Crono would not have stayed behind and acted like a sissy hiding in the castle dungeons or something, he MUST have done something to help his own Kingdom. I say OWN, becuase, as is stated at this thread's title: King Crono???.
He would not forsake his own Kingdom if he was the King.
But, it all comes down to wether Crono was King or not. If he wasn't, thing could be a lot different, like V_Translanka said, he could have been chillin at Ashtear Isle!