Is there a way to receive an alert or pop-up if a country of your choice goes to war? For instance, I am playing as Naples, and I'd love to receive a pop-up if one of my enemies is embroiled in a conflict elsewhere.
message settings are crucial for this game. First and foremost, and this might be the most neglected aspect of eu4. Hidden popup and pause messages that you can enable, that are off by default. I'll use some self made images to explain
In message settings select "all" tab and enable the popup and pause for every herald possible for *every option" in the "to me", "from me", "interesting" and "other" categories.. You can select unpause also for each popup you get. Then you will get the popup, but game won't pause. In many cases, this isn't good idea. Like when battle starts, when army arrives and whatnot.
Then, in the window above the map buttons select your interesting countries. The result is that you can perfectly see when a new king comes to power, or when an event takes place that kills an heir of a country.
If you want to fine-tune the message settings without disabling stuff you don't like:
You can enable certain message types only for interesting category and other types for other. Select your interesting countries, and you will get the messages selected in "interesting" tab for those nations you made "interesting". All the other nations will go under "other" category, and you can enable/disable whatever in that tab.
As for the messages in the message box: if you click "send to log" in the message settings along with popup, you get both. But I never use the message box above minimap, as its WAY to easy to miss stuff there. But if you want to use it, set the according message types to "log". Up to you then if you want popup and pause for that message type as well. This can be handy when you accidentally click a popup away you didn't read. If its important, and if you set important stuff to show in the log as well, it will also be listed there.
You will never miss anything important happening in the world again.
Example: Enable minor and major event popup and pause for interesting countries, for any event they get. Why? *If* that interesting country has 40+ king with a regency heir for example, and the target county gets the event that might kill their heir, you can see the outcome right after. No more AI unions without you having a clue. It netted me, yet again, a pu over a greater power, England... Their king was 40+, their heir got sick, and it left me opening to RM England and install a noble of my throne. King died year after, and I used the cb gain to start force union war. It took me 14 years of loans, looming bankruptcy and sky high war exhaustion when he died, but I won. All because I started eyeing them month after month, waiting for my chance in the game of thrones.
You can then later disable the popup and pause for message types you don't like (like what country gives military access to another etc) by changing it in the popup window (options at right bottom). Some important popup you should enable for popup and pause: when armies arrive at destination, when country gets new king, major and minor events for you, and for interesting countries (prepare to read ton of stuff, you *can* disable this but you might miss out on easy dynasty spread chances when a country' heir dies due to event), diplomats arriving back home, traders arriving back home, what country gets succession war with another, when a country starts integrating a vassal and so on and so forth.