Not sure whether you mean real life as in during WW2 or like during a real game.
The best way I’ve seen this strategy executed was using soviet infantry from Caucasus and a couple of tanks and planes hit Turkey round 5 and then the US takes Spain on its turn and if given the opportunity, lands all over the place to make a mess out of things. Then, the UK lands planes in Spain, and again if possible lands troops in southern France, or Normandy in order to prevent the German use of those factories or plane landing on their turn.
The British should have factories set up in the middle east by this point and should be slowly trickling in infantry into turkey to counter any axis offensives through the area, could also allow Britain to open up another front against Germany in the Balkans using their full IPC potential instead of having to use a portion on transports and a navy.
Overall I think violating neutrality as the allies is a good idea, but the allies have to be ready for it. The Americans should take Spain and be a nuisance of themselves, such as launching off to Norway, or Italy. Any distraction from killing Russia to Germany helps. The British can then pour tanks in through Turkey, and if the Germans have to invest significant resources into countering it, it may give Moscow an extra couple of rounds it needs to turn the tide of the war, or hold indefinitely. Also, the Americans cant spend all of their IPCs on Spain, there is a point where it is fruitless if Germany really chooses to crack down, also combining Spain with a Berlin Bank job kinda thing would be pretty nutty.
** all opinion, not a pro at the game i just spend a lot of time thinking about strategies id like to use **