The origional MARK I Sentinels were created by Bolivar Trask as a way to safley apprehend mutants (dispite the fact that his own son was a mutant). They were man-sized with big heads, and could fire a wide array of laser weapons, and had tough armor. However, after a few encounters, the X-men learned to kick thier butts. However, as a greater chalange, MASTER MOLD (a large sentinel with the ability to mass-produce regular Sentinels) still had to be defeated. It eventualy was, but Sentinles have the ability to learn from thier past mistakes, and bid thier time...
The MARK II Sentinels hit the scene, only now they were as tall as buildings and had the ability to counter mutant powers. They were-special mad for specific missions (immune to lightning, strong blows, etc) and each time out, they became tougher, and had more weapons at thier disposal. They even had the nerve to attack at Christmas once! They have often succeded in capturing and/or disabling the X-men, but the X-Men would always win because if they didn't, the comic would be cancelled and the Sentinels would have to find another job... In fact, in a far-off alternate futer, the Sentinels believed it to be in the best interest of humankind to subjugate America into a soverign Sentinel-run nation. I'd tell you to see my Days of Futer Past section, but um, it kinda sucks.
Now THIS is where the confusion comes in. The PRIME SENTINELS of Operation: Zero Tolerance are said to be mark IV, but there's never really been a mark III. The Mark III's would be tha MARK II's that invaded Manhattan durring Onslaught. Most people argue that each "Wave" is actualy a different mark of Sentinel, while most make a visual judgement. Some poeple make out the Phalanx to be Mark III. In either case, Eventualy Onslaught put is sway over the Sentinels, almost all of which were trashed by the Avengers, FF, and X-Men.
Then there are the "Prime Sentinels" that were used durring O:ZT. These sentinels are just normal human beings that, willingly or not, were subjected to nano-technology (No i'm not talking about annoying virtu-pets), that, in the presence of mutants, will change the person into a near-mindless killing robot. "near-mindless" as in there have been at least 2 times where a Prime Sentinel fought it's programing succesfully. And although O:ZT is no longer in "operation" (ba-da BUMP!), there are probably countless other humans out there with nano-technology waiting to be transform thier hosts into Sentinels.
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