IMHO, the best alliance you can get is Europe and USSR. Since the majority of USSR's population density is to the west, Europe and USSR can form an incredible defence network.
Europe is only easier to defend because it is tightly packed. However, if you could disable Europe's radar sites that are far away from the dense population, you can get an advantage. A radar site only needs one hit to go out, and the silos rely on them to be effective. If you spammed nukes and crushed the radar sites you'd put Europe at a massive disadvantage. Without an early warning system any nation is in trouble. Since Europe MUST concentrate its silos since it has such a tight population density. That means they'll probably have some radar sites left without any defence at all. If you take them out the silos will rely on radar close to their location, which will reduce their ability to defend effectively. Take out the remaining radar and they're flying blind... mostly. Also there are less places to place things in Europe, so you can be pretty sure that once you find one object, you've probably found the majority of them near by.
The only way you can really win is by overwhelming the air defence, or distracting it while taking out radar sites since it takes only one nuke to take down radar, but three to kill a silo. Otherwise, wait for them to launch and fire away.