Rapid deployment units are units that you can airdrop on a country you're at war with that isn't bordering you in order to setup a depot and airport to transport ground and/or air forces. Like tanks and mid range missile batteries. Good for higher war level ai and countries out of range from your airforce. Special forces are units you can airdrop in a non bordering country, basically their use is to paint the map while you use your air forces back home. They're not very good at attacking, only used to paint or take land. You'll need military transport units in order to airdrop/transport your forces. Special forces are very important in wars when you use your airforce and do not require an airport to land on, but only buy a few as they're expensive. I use 4 in my wars. Always remember to have them, and any other land units to return to base when you're done. When you start a war, the very first thing you should do is drop your special forces. Then attack the enemy just below 25 war points and have them paint the last 25. I hope this helps. Wars are still a great source of income if done properly.