Blackwater pulled guns on US soldiers

From MSNBC:

The colonel was furious. "Can you believe it? They actually drew their weapons on U.S. soldiers." He was describing a 2006 car accident, in which an SUV full of Blackwater operatives had crashed into a U.S. Army Humvee on a street in Baghdad's Green Zone. The colonel, who was involved in a follow-up investigation and spoke on the condition he not be named, said the Blackwater guards disarmed the U.S. Army soldiers and made them lie on the ground at gunpoint until they could disentangle the SUV. His account was confirmed by the head of another private security company.
...
Unlike nearly everyone else who enters the Green Zone, said an American soldier who guards a gate, Blackwater gunmen refuse to stop and clear their weapons of live ammunition once inside. One military contractor, who spoke anonymously for fear of retribution in his industry, recounted the story of a Blackwater operative who answered a Marine officer's order to put his pistol on safety when entering a base post office by saying, "This is my safety," and wiggling his trigger finger in the air. "Their attitude was, 'We're f---ing security; we don't have to answer to anybody'."

You pay with your tax dollars for Blackwater agents to pull their firearms on American servicemen and -women, disarm them, and make them lie flat in the streets of Baghdad.

Here's a couple of questions I have: what if the soldiers hadn't backed down? Would the mercenaries have shot them? How long until there IS a firefight between Blackwater and the US Army? How long until one of the mercenaries kills an American soldier? And what, exactly, could the US government, by whose laws Blackwater is not bound in Iraq, do about it?