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3.5.1 How do I move ground units between store, colonies and fleets?
3.5.2 How do I increase the number of ground units a planet can hold?
3.5.3 How do I invade an enemy colony?
3.5.4 Can I group combat units in battle?
3.5.5 Why do my defensive buildings not concentrate fire on the enemy?
3.5.6 Can I retreat?

3.5.1 How do I move ground units between store, colonies and fleets?

Fleets must be near a planet with a Military Spaceport to transfer vehicles. You can transfer vehicles between planets at any time. On the equipment screen (F4), select the planet or fleet, then the vehicle type. Click Del to move vehicles from planet/fleet to store, Add to move them from store to planet/fleet. There is no delay caused by moving vehicles between planets, meaning relatively small numbers of vehicles can be used to defend entire empires.

3.5.2 How do I increase the number of ground units a planet can hold?

Build additional Barracks, Fortresses and/or Strongholds on the planet. A planet without any such facilities can only hold a maximum of 8 defending ground units. These buildings add 4, 8 and 12 additional units per building, respectively.

3.5.3 How do I invade an enemy colony?

Attack the planet with a fleet that includes vehicles. Vehicles can be assigned to a fleet which contains a Flagship, while that fleet is close to a planet with a Military Spaceport. Three flagships, each equipped with cargo pods will increase the number of vehicles you can attack with. If the colony you attack is protected by orbital defenses or an enemy fleet, your fleet must first defeat those defenses. If your vehicle-carrying ships survived, you will automatically launch into a land combat, with the option to position your forces round the boundary of the colony. You need to destroy all the enemy vehicles (including non-combat units that may be hiding behind buildings, like Radar Cars) and all their military buildings (Barracks, Fortresses and similar), after which the colony will fall to you.

3.5.4 Can I group combat units in battle?

Select multiple units by dragging a marquee around the units with the mouse. Assign them to a group by pressing Shift + 1 to 9. Pressing 1 to 9 will reselect the group. Alternatively, select all units of the same type using: T (tanks), R (rocket cars), L (flagships), C (cruiser/destroyer) or F (fighters). "A" selects all units. After the first few battles you will get to choose your formation at the start of the battle. The "Group/Single" toggle displayed at this time will either group your fighters up into one space, or attempt to place fighters individually.

3.5.5 Why do my defensive buildings not concentrate fire on the enemy?

Shwatzy comments: "You can select your turrets and manually target the most dangerous tanks first."

3.5.6 Can I retreat?

You can retreat from space combat (use the button on the left hamd side of the space combat screen), although if the enemy fleet is faster it may continuously re-engage you. You cannot retreat from ground combat.