Draw a location over your world area and make this trigger.
Conditions:
-currrent player brings at least one men to locaiton world map
Actions:
-Center location 'unit' on men owned by current player at world map.
-Preserve trigger.
When he leaves that location (is warped to the battle area) then the conditions will no longer be true and the location will quit moveing. Thus it will stay where it was last centered.
Of course you can set the conditions backwards and do it this way
"current player brings exactly 0 men to location battle area"
Then when you move into the battle area the location will not center anymore.
1111th Post 
(last binary post i'll have until 10,000)