Select your country then on the top part, mouse over the icon that look like the 'set policy' mission (red square on screenshot). This will show you what options would be available at that time if you were to send your agents to 'set policy' in this country.



For common troubleshooting :

Want to start a War ?
You need the target country to be your rival
AND you can't be allied with any of their ally
for example : if USA is allied with UK and EU, and as the EU you want to war the UK you will first need to remove USA from your ally (or remove them from UK)

Want to Unify ?
You need the target country to be in federation with you (need to be ally too)
You need one country to have a claim on the other capital (either available by default like for EU member, or unlocked through Tech, see 'Great Nation' and further)
You need to wait for the diplomatic cooldown (you can see it in the previous screenshot, bottom part of the list, or directly from Relation->Manage)[/previewimg]

What condition aren't fulfilled (Thanks soldiers[PL])

If you want to know what condition are missing to War/Join Federation/Unify you can go into the Relation tab of your country, and mouse over any country flag. You'll need to fulfill any condition currently stated as 'failed' for the option to become available.