I believe the answer is:
- by making air power difficult to use, by preventing troop movements, impacting weaponry due to rain or snow, or any number of ways.
For example, in launching a missile projectile, the wind current could make the missile deviate from its original destination.
The waves and water current also could prevent the navy to reach its destination on time and might cause them to lose the War.
Weather patterns can often favor one side or the other or impact both sides in similar ways.