How do you send troops to defend against zombie attacks?

You can send troops to attack zombies that are in the world. The only time you need to defend from zombies attacks is the Dead Rising event. In this case you are defending your self and you can also reinforce other alliance members.

To send troops to alliance members as reinforcements you need to tap on their base in the world map. Then select the 'Reinforce' option. This lets you select which heroes and troops to send to them.