When you opponent is moved by religious zeal, it does not matter if you are an army.

Historically, the precedent was set by Hebrew zealots. They were men with religious zeal and they were willing to take on the top military weapon of their day - the Roman Legion.

So, what is it to men of religious zeal such as al-Shabab when they have to face African Union troops? They will war upon these troops:

http://www.aljazeera.com/news/africa/2014/08/al-shabab-attack-au-base-somalia-20148915423983656.html

In fact even it they were U.S. marines, al-Shabab will take them on as well . . . as they have done so.