When you have build a sufficiently large army, it is time to show them who is boss.

To attack an enemy, simply move you own units within range of the enemy and drag your own unit on top of the enemy unit. If nothing happens, the units are most likely too far apart.

You should be careful where and what you attack - grouped enemies stand stronger, and should be attacked from the "corners". Go for defensive support units first - that will make the rest of the job so much easier.