Crusades 

A series of religious wars fought mostly between the Christians (Roman Catholics) and the Muslims during the early part of the previous millennium.

They have been called Holy Wars but there was nothing Holy about them. Christians killed Muslims and Muslims killed Christians.  Neither side really won but both sides lost.  They were a rather effective form of population control.

Both the Christians and the Muslims actually worship the same Supreme Being even though they use different names for and have slightly different ideas about the Supreme Being.  Since there can only be one Supreme being they must worship the same Supreme being.  Why they had to kill each, and still do kill each other, over a name or an idea about the Supreme Being, is beyond my understanding, unless the killing has a lot more to do with gaining land, money and power than it has to do with God.  Could it be that Expansionist Religions like Christianity and Islam may be more concerned with land, money and power then they are with worshipping the Supreme Being?  Also God, who created everything, needs no money or help in any form.  Nor has the all-powerful God ever needed a big fancy building built by little men that is called a Church, Mosque, or Temple.  God already owns everything.  God needs no money.  God needs no help.

The various Popes of the Roman Catholic Church, in their desire to gain control of the lands occupied by the Muslims, caused several kingdoms to spend so much money fighting these worthless Crusades that they became poverty stricken.  Also, these countries lost a lot of their men is the stupid attempt to gain more land for the Popes to rule over. The common people of these countries suffered greatly because of the Pope's desire for more land for the Popes to govern.