day no. 15,148: combined arms

"The use of combined arms is a key means of gaining advantage. It is based on the idea of presenting the enemy not merely with a problem, but with a dilemma—a no-win situation. We combine supporting arms, organic fires, and maneuver in such a way that any action the enemy takes to avoid one threat makes him more vulnerable to another. For example, an entrenched enemy should discover that if he stays hunkered down in fighting holes, Marine artillery and air will blast him out. If he comes out to attack, Marine infantry will cut him down. If he tries to retreat, Marine armor and airpower will pursue him to his destruction. That is combined arms." -- MCDP 1-3: Tactics

When playing tic-tac-toe, the strategy is to force the hand of your opponent into two equally bad options.

If you are "X" you want to end up in these type of situations. 
If you are "O" you want to avoid ending up in these type of situations.

"O" has a choice to make in the above scenarios, but they are equally bad. You can only prevent "X" from defeating you one of two ways and to pick to defend that flank is to surrender the other.

This is the same strategy behind combined arms. If the enemy is only ever focused on shutting down one of your assaults, it opens them up to be defeated by another one of your assaults. 

On the other hand, beware that you don't fall prey to the same tactic. Our enemy knows how tic-tac-toe is supposed to go as well and is in no hurry to submit to your strategy passively and is furthermore actively attempting to place you into the compromised position leaving you with only the choice of how to surrender.