This command starts an annex with the specified country tag (an annex claims their territory for your country).
annex [country tag / 'all']
|Country Tag / 'all'||The country tag of the country you wish to annex. Specify 'all' to annex all countries.|
Executing the above command would start an annex with Germany (GER), claiming their territory for your country.
Executing the above command would annex all countries, effectively making your country the owner of all territory in the game.