Annexation

Annexation is a mechanic which allows an attacker to conquer territory without having to use troops or time.

Details

Annexation is possible as long as the following conditions are met:

1. The defending territory has a single owner.
2. The defending territory is not wilderness (it can be owned by a human, bot, or nation).
3. The defending territory is fully surrounded by the attacker's borders (meaning it cannot have any borders the ocean, a river, or another player or bot). It may have borders with a lake, as long as it surrounds the lake, and the attacker surrounds it.

Once all of these conditions are met, if the attacker attacks the surrounded territory, it (almost) instantly becomes parts of the attacker's territory with no troop loss.

You do not have to conquer a whole nation/player in order to conquer a smaller surrounded part of their territory.

Strategy

Early Game

Rushing to surround (and then annex) bots is a very quick and easy way to gain money and territory.

General

Using several low percentage naval invasions on a coastal region which you have otherwise surrounded allows for saving on troops and time by only having to fight through a small amount of tiles to surround and then easily capture the given territory.

If a territory consists of easy terrain (plains) surrounding difficult terrain (mountains), it may be possible to annex this territory simply by your troops surrounding it naturally given plains are easier to capture.

Sometimes, you may be able to surround territory of a larger player, or nation, which has more territory elsewhere on the map. If such a situation occurs and you share another border with the defending nation, it may be a good idea to attack with a low troop attack percentage in order to not waste troops on slow land warfare. Another option is to retreat the troops manually.

Miscellaneous

If you have a significant troop advantage on a fully surrounded player, nation or bot, and you do not need gold in that exact moment, and you do not have significant trade, it may make sense to not conquer them right away, as the longer they are alive, the more gold they can produce. Caution should be applied when using this strategy, as if you forget about the territory, and it gains a foothold elsewhere, or you lose enough troops in another attack, it may attack you with enough force to stop being surrounded and it may threaten you. Also note that bots have a slower gold production rate than players and nations (only 500 gold per second, compared to the standard 1000 gold per second).