Basically, Italy, Japan, Hungary, Romania, and Bulgaria ally with Germany earlier than they did OTL. (In Hungary's case instead of a puppet Slovakia, they annex the Slovakians when the Germans annex the Czechs.)
It's a very openly known, if you attack one of us you attack all of us thing. (No Mussolini sitting on the sidelines and waiting until France is already beaten to get involved.)
------------------
How would this effect the Pre-WW2 period?
Would it cause further hesitance, and thus prolong the period of appeasement? (Since it's more than "We just have to worry about Germany." now?)
Could this have instead been too much? (IE: Just Germany is ok but all these nations getting uppity? Better show em what for.)