In our games, the Allies have never attacked the strict neutrals. I guess we get in the mindset “Hey, we’re the Allies, the good guys and we aren’t going to invade anyone’s neutrality”.
We did have two games where the Axis (mainly Germany) tried the “round 3 neutral crush” hitting Sweeden, Spain and Turkey all at the same time. In one game, the Axis won and the other they lost. I don’t remember there being a lot of difference between those two games except for dice rolls and minor planning differences.
In the Axis won game, the Italian and Allied fleets wiped each other out in the Med and the Italian army was killed off in Africa. Every time Italy put transports in the water, UK fighters flew out from Malta and sank them. So, once Turkey was taken, Italy simply built tanks and mechs then went rolling into the Middle East and got Egypt anyway. Not only that, but they threatened Calcutta, which allowed Japan to more easily take India. Sweeden pretty much locked in the German NO for Sweedish Iron Ore. Spain ended up being a back and forth battle between US and German forces, which kept the US from helping either Britain or Russia and gave Japan a little more breathing space in the Pacific. Also, the US invested too much in grabbing up the South American countries all at once and didn’t send enough transports down to bring all those new men back to the front.
In the Allied won game, the US still got the South American countries, but more leisurely. They landed in Brazil and activated those units. Then the Brazilians activated Venezuela and Argentina. Then Argentina activated Chile. Then all units moved back to Brazil and US sent a few transports. It took a few rounds, but suddenly USA had several more transports filled with men to land in Europe. The Turkey battle went badly for Germany and Italy ended up taking Turkey instead, which was then taken, and kept, by Russia. So instead of the Axis having a gateway to the Middle East, now Russia had an extra avenue into Southern Europe. The US held onto Spain and was then able to liberate London after Germany took it. The troops used in Sweeden left Finland weak so Russia was able to bully them out of the way and took Norway, which cancelled the German Sweedish Iron Ore NO after all.
So, in one game, it seemed like a brilliant strategy that worked very well. In the other, it cost too many Axis resources and preperation that wasn’t being used on their true enemies, who took full advantage of that.