Guide by Big Al from Oz at http://warbookforums.com/smf/index.php?topic=380.0

It has taken me a long time to work out the single most important strategy in WarBook.

The in game help says,
"Be careful not to explore too much, as you can overextend and provide easy prey for aggressive kingdoms."

Whether by exploration, conquest or landgrab, what is overextention?

You have army size N at acres A, you grow to army size N at acres A+x.
You are now weaker for your size than you were before.
This is not inevitable.
Because you also have money.

You have army N, acres A and gold G, you grow to army N+n, acres A+x, gold G-y.
You are now exactly as strong at A+x as you were at A -- that's true progress.
Your level of income drives your level of sustainable growth.

The trick is being efficient and realistic about what kind of army you have at any acreage.
The proportions should be the same. Growth in WarBook is exponential,
in other words, the bigger you are, the faster you grow, but the proportions remain the same.

Where normal people go wrong.
Perhaps I'm just dumb, but I prefer to think I'm normal.
When I started, I kept trying to grow, because I wanted more income to grow faster.
That's not dumb, it's right.
The problem is growing faster than you can afford to fill your new land with troops.
Sure, at higher acreage you've more money so can buy the troops faster,
but there are two serious issues:
1. understrength kingdoms are "easy prey" (don't believe me, try it, you'll see)
2. if you move from understrength up to even more understrength,
you have to "fill up" the old acreage, plus the new acreage as well.

Even full strength kingdoms are prey to someone, you'll never be 100% safe.
So the issue is minimizing losses not stopping them altogether.

The way the combat system works, you lose more land than army in battles.
In other words, losing battles actually makes you stronger at the new (lower) level.
This means, if you were fully occupying your land when you lost,
you've already got a head start on your next expansion.

The trick is to expand more than you lose. Everyone has the same issue.
Spend your resources on fighting battles you can win,
not on trying to pursuade others not to attack you in a war game.
For some reason this doesn't seem to work very often.

Work out your ideal army per acre, then clone it.
When you can't afford to do it for any more acres, stop.
Even if you lose half your land, you will lose a lot less than half your army.
You'll be all set to recapture that land when you come back.
Recapture is a lot easier than reconquest IF you avoid over expansion.

But I should say, overexpansion is fun, and often leaves more income,
but this is only because you effectively have lower army
and less strength than others at the same acreage.
True strength is acreage plus army.
Army size is an investment of money that has genuine positive consequences.
To invest in army, while staying at a given acreage, is genuine progress,
and, essentially will always be an option until you learn to stop overexpanding.