Thanks Martin, I misunderstood the armistice condition there.
This could be done via a naval invasion but you can also accomplish the same thing by simply moving four units across the Messina straits (two landings markers needs to be available).
Ok, I just went back to the rules and I think got it now. I did not realise you could just move, provided you had the landings markers available. I assume you can only cross from the three hexes adjacent to the straight on the Sicily side of the map.
Anyway, we did not get into that situation as the Allied stalled and never made it beyond the Limited air cover line.
Next time, I'll try another scenario or the short campaign. Excellent game!