Well as you know, both the E3 and the Games Com Demo's gave the exact date of the missions, excluding Slaughterhouse. But here's the current timeline of the missions we have
1st: Slaughterhouse - January 31, 1968 
2nd: Victor Charlie - February 12, 1968 
3rd: Payback - February 12, 1968 
4th: WMD - March 5, 1968
So i can say that the theory on "WMD being the first mission" is busted. The Firstmost mission up to date is Slaughterhouse.
-  - The Tet Offensive began in January 31st. Which happens to be the main battle of Slaughterhouse
-  - Both Victor Charlie and Payback happen to be on the same day, but Payback is atleast a few hours later than Victor Charlie. As you can see on the E3 demo, they're both on the same period of the day.
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