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According to NATO Vehicle Guide, Canadian 4th Mech Brigade crossed the East German border on 12/9 and entered combat on 12/12.

British Army
1st AD: 12/10/96, 12/12/96
2nd AD: 12/10/96, 12/12/96
3rd AD: 12/9/96, 12/10/96

5th Royal Inniskilling Dragoon Guards: 12/96, actual date of first combat unknown
The Royal Hussars: 12/96, actual date of first combat unknown

So we basically have the equivalent of four more divisions entering combat between 12/10 and 12/12. When one takes into account the additional three US heavy divisions that also enter combat in this timeframe, it seems likely that a major operation exploiting one or more breakthroughs by the first echelon of the NATO attack beginning 12/6 and expanding through 12/8.

Without knowing where the front line trace was leading up to the US offensive, it’s hard to do anything more than conjecture as to the shape of things. However, it seems reasonable to believe that the West Germans would have made every effort to keep their lines relatively straight as they fell back so as to avoid presenting tempting salients. SACEUR would have wanted to exploit the armor friendliness of the North German Plain for offensive operations while exploiting the defensive advantages offered by the terrain in the south. This may help explain the gaps between US Army units crossing the frontier and their first action. I imagine UD forces attacking on a four-division front through Magdeburg with the intent of having the right flank of the offensive skirt the northwest shoulder of greater Berlin. The second echelon, or a portion thereof, would have continued the offensive on a northeasterly axis past Berlin and to the Oder River. Pact forces left in Mecklenburg would have been cut off from resupply except by air. Once these forces were dealt with, Anglo-American divisions could have been reoriented to face south along an arc north of Berlin. An attack south along the left bank of the Oder would have been able to use the river to protect the left flank of the drive while inviting the Pact to allow some of its forces to be encircled at Berlin. In general terms, NATO (including West German forces) might then have pressed south to squeeze the Pact out of East Germany.