As a very quick middle-game survival guide, here are bad situations, roughly in order from worst to least-worst. I call these "red flags" because they indicate a danger. You should generally avoid these situations unless you can make something even worse happen to your opponent. Contrariwise, it's good if your opponent has one of these problems.

During the middle game, you will continue to try to get more and bigger ships than your opponent, but now your fleet is too big to fit in your homeworld. The other systems where you keep ships are "colonies." Keeping your colonies safe and attacking enemy colonies is a vital skill. Many of the same principles apply to defending and attacking homeworlds, except that homeworlds shouldn't be evacuated in the event of an attack.

This section is mostly written for the defender. When you are attacking, look for ways that your opponent has failed to take this advice and exploit it!

One of the strategic tips in any Homeworlds strategy guide is, "When your opponent gets red, get red yourself." This is good advice. It often doesn't even matter much where the red ships are. If your opponent has a red ship anywhere, you need to get one anywhere that you can. Otherwise, your opponent can move any ship to a system where you have ships and you won't be able to capture it (unless the star is red). Your opponent will then be able to capture your ship(s) by sacrificing their red ship.

A single red ship can simultaneously deter invasions at all of your systems. Regardless of which system your opponent chooses to invade, you can sacrifice the red ship and capture the invading ship. Consequently, your opponent may choose not to attempt any invasions. I call this the "deterrent principle."

Similarly, if your opponent lacks red, your red ship can be used to power an invasion of any of their systems.

...but a red ship can only be sacrificed once

A single red ship can deter attacks for quite a while, but remember that a single red ship can only be sacrificed once. If your opponent launches an invasion that forces you to sacrifice your only red ship, then you don't have a red ship anymore. You may now be vulnerable to a second invasion where your opponent has red ships to sacrifice but you do not. If that second invasion is at your homeworld, you're likely to lose the game.

If you have a material advantage, you should often be willing to cause catastrophes or force mutual sacrifices that trim down both fleets by similar amounts. A one-ship advantage is a bigger deal when both fleets are small than when they are large.

Contrariwise, when you have a material disadvantage, you should build ships and not worry too much if your opponent builds the same amount. A one-ship disadvantage shrinks (proportionately) if both fleets grow at the same rate.

If you have more large ships than your opponent, try to harass colonies before they have time to get another large ship. Move that big ship into an enemy system with lots of smaller ships and no big ship defending. Attack any ships that don't manage to escape. Be careful of catastrophes, which are often the best defense against larger ships.

If you have a large-ship disadvantage, spread your fleet out. A lone ship that can run away is an unattractive target for an invading large. Ideally, every ship should be be either

• In a system with a large defender
• Or in a system with no other ships, but with an escape plan (e.g., the ship or star is yellow).

When a colony gets invaded, you have more options than when your home is invaded. Abandoning your homeworld is not an option (unless you can force a draw), but it's often worthwhile to evacuate a threatened colony.

When your colony gets invaded, figure out what kind of threat the invader(s) pose. Will they capture your ships? Or are they preparing to overpopulate your ships, or even the star itself? Or maybe they just wanted to reposition their ship and they put it in your colony to add confusion, creating an empty threat.

If the threat is real, here are the usual options:

• Capture the invader
  If the invader was just the first wave of an overpopulation attempt, you may have just wasted your time (and a red ship if you sacrificed).
• Evacuate (run away)
  A lone ship is usually easy to evacuate (moving away, green teleporting, or maybe you have a y1 worth sacrificing elsewhere)
  Multiple ships can escape via yellow sacrifice, so much the better if one of those is a threatened ship.
  Any single ship can at least avoid capture by sacrificing itself, even if there's nothing you want to do with the action.
• Overpopulate the invader
  If their threat is capturing your ships, maybe you can cost-effectively overpopulate their ship.
• Overpopulate the star
  If their ship is worth more than all of your ships in the system, consider blowing the whole place!
• Counterattack
  Your probably know what your opponent plans to do with their next turn (capture your ships or overpopulate them), so you may want to create a threat elsewhere that they will want to respond to first (e.g., poise yourself to build a big ship or invade a system of theirs). If you keep creating threats, they may never get to finish the invasion.
  If the invading ship is large, it may have just left a colony where it was the only large defender. If so, consider invading that colony, especially if their colony has more ships you could capture than the colony of yours that just got invaded.
• Ignore the threat
  Sometimes you can get something more valuable elsewhere. For example, if you can build a couple of large ships, you may want to do that rather than rescuing smaller colony ships.
  

Simultaneous multi-colony invasions are like double-check. It's difficult to deal with multiple threats.

When you go on the offensive, just look for colonies where your opponent has one of the weaknesses discussed in the defense section. Then take advantage!