The Posse Comitatus Act is supposed to keep images like "tank man" in Tiananmen Square from ever developing in America.
Not that the Act was ever intended to help people, it is actually rooted in tyranny (like so many other laws.) Posse Comitatus was a compromise by Northern States to fraudulently elect Rutherford B. Hayes as President over Southern objection. To placate the South, federal troops safeguarding recently emancipated blacks were withdrawn and the Act passed as a guarantee against Northern interference in the subsequent "Jim Crow" laws.
The Bush Administration tried and failed to overturn Posse Comitatus, but continue to act as if they succeeded. They have managed to muddy the legal waters concerning the use of military force in the "homeland." Having their foot in the door to martial law, one wonders where this may lead in the future.
In October 2008, just as the financial crisis was coming to bear, an active US Army Brigade Combat Team was assigned domestic duty. Federal troops have also been used in mock disaster drills in support of local police. On December 10th, 2008 the US Marine Corps deployed military police alongside California Highway Patrol to set up drunk driver random traffic stops. The Marines were supposedly along just for training purposes.
What does all this have to do with Tiananmen Square? Tank Man's picture is from June 4th, 1989 -- more than a month after students occupied the plaza and just as events turned deadly. The local People's Liberation Army units refused to move against the protesters (many were friends and relatives), so units from the 27th and 28th PLA were brought into Beijing from outside provinces. These units were made up of formerly poor, ignorant farmers who had no problem pulling the trigger on a bunch of whiney rich kids.
Red China had no Posse Comitatus to protect its people. Some of the Army rebelled, but there were plenty of outsiders to do the government's dirty work.
If you're wondering about how inclined our military is to pull the trigger on fellow Americans, consider that five percent of the armed forces are now foreigners. As incentive to join, foreigners are offered citizenship in return for service. It won't be "Americans" pulling the triggers on us when martial law comes to pass.