You play US, I play Wehr.
You want to go for a quick M8 and therefore get the Supply Yard and Motor Pool as fast as possible. I go for Kriegbarracks <---- I can get counters to your M8 now. Your response should be to:
1. Cancel Supply Yard if it's not completed.
2. Cancel Motor Pool.
In any case, you should go for BARs. That will make my Schrecks and AT-guns useless.
You have to understand the factions to really know what to do in a given situation. But disregarding that, just think logically. "Ok so Campaigner has Grenadiers. That means LMG 42s and Panzerschrecks + he can get AT-guns, Mortars and Halftracks. It also means he has to either get T3 or T4 since it's too expensive to get both" <--- BARs and supportunits (Snipers and/or Mortars) will work greatly until I would get Pumas or Stugs out.