What Is Defending?
In NationStates, the military aspect has been invented by players, so the admins have adapted the game to suit it. Founderless regions are regions without a founder, and when a founder dies the regional controls swap to a delegate who can then eject and ban people from the region and password it.
Invaders seek to take control of regions by sending lots of WA (World Assembly) nations into founderless regions and taking the delegacy by endorsing an invader lead. Typically invaders leave a region after a few days, but other invaders have been known to destroy regions, often so they can refound the region (recreate it with them as a founder) and therefore have complete control over it and make the 'natives' join their region and army etc.
As defenders we seek to defend regions or 'counterinvade' as some people put it. We move in WAs to support the native delegate against an invasion or we endorse a defender lead so we can take the delegacy and ban and eject the invaders. This is called a defence.
We also sometimes have to liberate regions that are already under the control of invaders. This is a lot harder, you have to find the update time (the time when the server counts endorsements and changes the delegacy which happens daily). Then we have to move as many WAs in as possible in the short minute or two minute slot before update so that the invader doesnt have time to banject us.
Its great fun and there are some much more advanced tactics that are used which hopefully you will pick up on along the way. ^_^