having a reaction force
OK, that just advanced my 1939 Allied thinking in a major way, in a general sense. I think my usual UK set-up might change a little.
I generally set up the French TRS in Syria.
In the case of Algieria though, things are sticky. Algiers itself can't be invaded. I had been looking at the fact that the rail line to it can be blocked by an invasion west of the city. So presuming an Italian landing on Impulse #3, the Allies can land a unit in Algiers from the sea. The problem is they may have to fight through Italian forces at sea in the West Med, possibly including a NAV in 4 becomes 5 box. So CV cover will be needed. A bit tricky if 'reaction' means Combined impulses for the Allies.
Algiers is nice to have for the Axis for sure. But it puts them a longer way from Oran, and Oran is pretty important to the CW = they will set up a front in NW Algeria.
If the Italians land in Oran itself, an Allied reaction force can't do much unless it includes an HQ - unlikely unless the CW set up a TRS in Egypt. By the time the CW could march up and counterattack Oran, things like an Italian HQ-A and a MECH could be sitting there, as they couldn't sail straight into Algiers until the French march out. The 2 French INF, one from Syria and one in Algeria, could do the job though. 3 defense vs anywhere from 7 to 11 attack with large variables on shore bombardment available depending on results at sea.
It is coming back to me now finally. I set up the INF in Algeria in the clear hex east of Oran, covering Oran and another landing hex with ZoCs at least. The Italians could get between that unit and Algiers but could not take Algiers before an Allied impulse, nor land in Oran for free.