The Global Diplomacy Status menu shows current major diplomatic relations between all Nations. You can see another Nation's Alliance members, or who they are at War with, as well as all Vassals and Overlords. All other diplomatic statuses are not shown in this menu, and thus are not visible to all players.

If a Nation is at War, you'll see either an [A] or [D] before the names of the Nations they are warring with. This shows which Nations were the aggressors (the ones who declared the war) and which ones are on the defending side. (This is purely informational - there is no tangible difference between the attacking and defending sides of a War.)