This is the theater that the Germans seemed to have taken much better lessons from than the Italians. Example first us of the 88mm Flak in an anti Armour role was in Late SCW Could Imagine France closing off the Sea to Nationalist's And the War being over faster.
If there are French forces and say getting some MS406 fighters against the Nationalist Bombers and the CR32s not able to do a thing about them, Might be the Italians start getting some lessons. And it was a better match here against Bf109C with 4 MGs than the later and Faster Bf109E it would only face over France in 1940. Alos Tactics that work against the Stubby Russian fighters might not work against other types
Lessons here,
French discover Dive and climb Fighting tactics against Bi planes and allot of respect for the German fighters
Germans get to think the French have some fighting spirit and there Bombers are vulnerable to Cannon armed Fighters they cannot out run
Italians. Mono planes are the way to go and a Aircraft mounted Cannon is a nasty thing
Tanks the Doctrine of the 2 man light Tank with the 1 man Turret might show it's failing early enough to perhaps change French Tank designs, Certainly the need for a good Anti Armour weapon on your Tanks, Char B heavy Tank might also teach the Italians a thing or two also the Hard way
I see it more likely the French going in, Hard to see the British there. Maybe a spat between France and Britain. Nothing major, The other thing holding France out was it was against the Ideal of the league of Nations, Which it was a major force behind. Bit hard getting them in.
Possibly a real stupid Navigation error on the part of some Nationalist Bomber in a night attack Dropping bombs over the boarder, that is possibly enough for France to say enough is enough, Bit hard to think why they would be there close to boarder, But silly mistakes happen, Disoriented Pilots just look at a few German Night fighters Landing in England, when they got lost, Thinking it was a french airfield.
I see Tactical lessons being learned all over the place Strategic ones less so