BRUSSELS/WARSAW—Polish security forces fired water cannon at rock-throwing migrants on the border with Belarus on Nov. 16, and NATO reiterated its support for Warsaw in a crisis that has left thousands stranded on the frontier in icy temperatures. Video footage released by Polish authorities showed migrants also throwing bottles and logs across a barbed-wire border fence, and using sticks to try to break through. Seven police were hurt in the violence, the latest in a crisis the European Union says is orchestrated by Belarus—an ally of Russia—in retaliation for EU sanctions imposed over a crackdown on political protests, a charge that Minsk denies. Up to 4,000 migrants, mostly from Iraq and Afghanistan, are now waiting in freezing forests on what is not only Poland’s frontier but is also the external border of the EU and NATO, the Western military alliance. “We are deeply concerned about the way the (Belarusian leader …