>For them to be able to easily overrun towns, take them over and move on to the next town.<
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I heard that many of the Iraqi Sunnis deserted and joined ISIS and that the Shia were slaughtered?
These acts of terror only weakened the resistance in subsequent towns until it became a rout for the Iraqi army.
It is expected that, now that ISIS has reached Shia area, the latter will put up a more serious fight especially now that they will receive help from Iran. They will have to put up resistance in order to survive.
There is no room in ISIS for the Shiites.
1700 Shia were slaughtered at least.