Which factor limits the ability of the united nations to supply peacekeeping troops in the case of armed conflict?

The factor that limits the ability of the united nations to supply peacekeeping troops in the case of armed conflict is: Member nations must volunteer to provide the troops.

In a term of armed conflicts, united nations HAVE TO SHOW neutral stand to all the countries involved, so they couldn't give direct order to send soldier for one side only. In order to give help, the members have to voluntarily provide the help without any force.