Master of Orion: Conquer the Stars/Expansion

Cutoff the opponent
You build up a fleet of the strongest war ships available. Example: 3 each Destroyers

Then you send the fleet to an unprotected system - typically adjacent to your territory. Send each ship to an outbound warp point and set them into guard mode.

Afterwards, send a Colony Ship and Space Factory to the guarded system. Colonize the best planet. Get the Space Factory to build Military Outposts at each guarded warp point.

Once the outbound warp points are protected by a Military Outpost, gather up your war fleet and repeat.

If the lane to this system is protected at the last system, scrap the Military Outpost as it will no longer be needed.