In all 4X space empire games I encountered the same annoying characteristic, the best strategy (at least against AI opponent) was to group all/most of your ships and raid through the galaxy with this monster. How to fix it?

EDIT: I'm not talking about games like Civilization (where there are actual stacks and they fight on the main map) but about moving fleets on the galaxy map like in Master of Orion (where the combat takes place on another tactical map).

I have seen solutions like slow ships and long distances (so you needed to keep fleets in various parts of the map since you were unable to move them quick) and limits to number of ships in a fleet (Endless Space). Both had their serious drawbacks...

The usual solution seems to be area of effect based weapons.

Ineffective against single targets but can be very effective against larger groups.

Try to come up with a similar mechanism maybe.

As in life, overwhelming force is an extremely difficult strategy to counter (unless you're good at guerilla tactics and you've got time on your side). I'd think about the logistics of moving your entire fleet (I'm assuming 4X = space games to you based on your post) and making sure the player has the resources and supplies needed to keep them flying, fed, and armed to the teeth.

Then, once the other opponents know the big guys are out killing stuff, have them all attack the home base.

The usual solution seems to be area of effect based weapons.

Ineffective against single targets but can be very effective against larger groups.

Try to come up with a similar mechanism maybe.

No, no, I meant a map of the galaxy. Like Master of Orion, Endless Space, etc. Moving fleets between planets, not combat (which takes place when 2 fleets meet and it opens a separate tractical map or something similar).

I guess I should edit my first post to make it less confusing

As in life, overwhelming force is an extremely difficult strategy to counter (unless you're good at guerilla tactics and you've got time on your side). I'd think about the logistics of moving your entire fleet (I'm assuming 4X = space games to you based on your post) and making sure the player has the resources and supplies needed to keep them flying, fed, and armed to the teeth.

Then, once the other opponents know the big guys are out killing stuff, have them all attack the home base.

No, it's not about overwhelming force. It's about making the player to attack 3 enemy fleets/planets at once instead of just 1. To make it not desirable to make just one big fleet that go around and fights. To promote making severl smaller fleets (in short a player that make one big fleet and fly around winning all fights due to numbers should lose (on strategic level) with another player that made 3 much weaker fleets and target separate planets).

Beyond what GoCatGo said, there's also the march of progress to consider. If you attack only one planet at a time, your enemies on all the other planets have a chance to march up their tech trees and be much tougher when you reach them. Therefore making it an advantage to attack as many planets as you can safely.

EDIT: I'm not talking about games like Civilization (where there are actual stacks and they fight on the main map) but about moving fleets on the galaxy map like in Master of Orion (where the combat takes place on another tactical map).

I've not played those games so my previous comment may be unhelpful, uninformed, and useless. That has never stopped me before, so I'll continue by adding:

Have them eaten by Warp monsters a la 40K! "We started out strong, but most of our fleet never arrived."

Tower-based strategy games don't have fleets, they have perimeters instead. Example: http://www.kongregate.com/games/duerig/shattered-colony-the-survivors (free zombie strategy game on Kongregate - buggy and not fully developed but fun and illustrates the concept.

I'm playing Neptunes Pride 2 at the moment. If you group all your ships into a single army, sure, you can take any planet you want. But in the meantime, your enemy can split their ships into 4 smaller armies and take 4 of your undefended planets. When you move your big army to take one back, they lose one planet, but they also move on and gain 4 more...

As long as the war is taking place over a fairly large front, you're naturally forced to split up your armies.

You could also introduce some artificial mechanics to discourage large armies -- in the tactical fights, have larger groups be less efficient / take more losses (such as the area of effect idea above), or, in the strategy map, have larger groups move slower than smaller groups (e.g. tell player's that there's overhead in getting a whole army to synch up their jump/arrival times to warp simultaneously, etc).

Europa Universallis has a maximum support limit on each space exceed it causes your forces to weaken through attrition. Also army moral is a key combat stat and much larger army can be defeated if its moral is low from constant fighting. You could have something large fleets exeeding your supply range and capacity could gradually weaken. damage, and max hp, and shields might decrease every turn, they might run out of missiles, or ammunition.

You could have a logistics stat which determines the max fleet sizes or the maximum number of ships you can own.

Planets and colonies might be highly vulnerable unless defended ala Risk. Forcing you to leave ships behind to defend or have newly conquered territory taken away very easily as you front expands.

The speed of the fleet might be based on its size. 1 small ship moves 10 parsec a turn. 10 large ships move 1 parsec a turn.

Scorched earth anti colony weapons are devastatingly effective. 1 fast attack ship making it to a colony is enough to destroy it.

MOO2 had telepathic control which was my main tactic. A cruiser or bigger can instantly convert a planet to your side once its defences are destroyed.

Privateers/partial knowledge raiding/destroying can be done as a stealth action. If you have a small and powerful enough force an enemy colony can be raided or destroyed with out the enemy knowing who attacked them. Based on distance from their capital, the size of the attack force, and number of turns spent attacking.