In Unaya, all countries begin with a treaty with every other country, so "declaring war" is done by breaking the treaty. To do so, click on a sector belonging to that country, then click the country's name to view its information. You should then see a "Break Treaty" link.
Just keep in mind that there are significant pros and cons to breaking a treaty in a Declaration of War world. It's a very different dynamic to the game, so I would recommend carefully considering that step!
