Or perhaps, how about decreasing the maximum number of turns on hand to half the normal, and transferring the remaining turns into stored turns on declaration of war.

E.g. 120(0) becomes 60(60). 120(40) becomes 60(100). And you'd only be allowed to hold 60 turns on hand at any one time while the war declaration is in effect (granted yes you should still be allowed to get bonuses such as the 18hour bonus and bonus points in turns which could easily push the 60 turns to around 66-70 or more turns on hand).

Thoughts?