I was initially thinking that the Spanish would use Guerrilla markers (GM) to nullify the better French cards, Cavalry Charge and such forth. However, it occurs to me after 4-5 plays that the Spanish might be better off hoarding their GMs for a while, and then going for as many turns in a row as they can manage, with a good chance of no French reply.
So for example, with 4 Activate Centre and 3 GM markers, the Spanish could try and take 4 Turns in a row in the Centre with the French just getting Battle back attacks. This seems like it could be a game changer of a strategy.
I've seen it happen a time or two. For the Spanish to get, say, three turns in a row (barring Sabres, of course,) they only need two Guerilla markers. That's often attainable as the Spanish, especially if you're willing to drop a Counterattack card on a French-played Scout card to pick up a marker.