Your country and the enemy country are at war. Declaring war instantly gives the war relation to both countries instead of being interchangeable. Any units that meet will do battle. Any units within bombardment range are subject to damage, and any ground unit reaching an enemy province center unopposed will result in capture of the province. There is an automatic trade embargo on both sides. Declaring war decreases your popularity with every country. A small morale penalty is incurred for being at war.