There seems to be a new trigger and command in HOI 1.06. The underattack trigger, and the protect tag. Using under_attack = POL, for example, would trigger if Poland is fighting a defensive war. The defend command allows a country to enter a military alliance with another, even if they're not part of one of the three main alliances. For example, if Yugoslavia has an event command = { type = protect which = BUL }, it would enter a militayr alliance with Bulgaria against whoever Buglaria was at war with. ie Bulgaria and Yugoslavia are at war with Hungary. And none of these three countries would need to be part of any main alliance.
This is VERY powerful. Using this, you can similate lots of things in MDS. For example, you can trigger spheres for NATO. If any of the NATO members are attacked, you can have all other NATO members defend that country, and you'll be able to do this within the framework of the existing CotW, Europe, and China allliance system.
So while Germany wouldn't be a member of US's alliance, but if it gets attacked, a NATO member like the US will defend it.
Using this new system, sonme ideas you can include:
-The NATO defensive alliance, NATO members will defend one another
-US can defend an attacked Mexico or Canada
-US can exert a sphere of influence over South America
-Russia can defend the 'stans and eastern Eurupean countries if they get attacked
-Taiwan isn't part of China alliance, but if a third country attacks Taiwan, China can declare war on that third country
-If a European counry's foreign colony gets attacked, its former colonizier could enter a defenisve alliance with it, ie France could defend an attacked Ivory Coast