Became Pharaoh as Seti

[Commander Liberty]

And the whole world declared war on me about 2 turns later wtf?

It's such a dumb mechanic, enemies walk next to each other they don't even engage, it's literally me against the whole map, what kind of trash game is that? lol

[Monteizo]

Two possibilities, if there are just standard wars then you just failed in diplomacy and it has nothing to do with becoming Pharaoh, just play the diplomatic game better, make friends, get trade deals, don't look too weak, you know standard stuff. Or there is a civil war, and in that case it can be trickier to prevent depending on the situation but very much possible as a faction can only start a civil war if they have enough legitimacy compared to the current ruler (after it has started any faction within the tradition can join though so best to prevent it from starting to begin with if possible).

Now I did not get any civil wars in my recent Seti campaign, because after I became Pharaoh (at turn 3, Merneptah died very quickly in that campaign) I checked who was the potential problems, it turned to be Khaemwaset (the new guy ruling Merneptahs faction) and Amenmesse. And Khaemwaset was not a problem as I named him heir, thereby preventing him from starting anything as the named heir can't start civil wars and then Amenmesse could not keep up in legitimacy so could in the end not start anything either (but if he does better in your campaign avoiding him starting a civil war will be harder).

When it came to getting the legitimacy, after having taken out the starting opponent I went after Khent-Min (despite them actually liking me, but as I already had a presence in the province as your starting opponents capital is a minor settlement there it is a fairly natural route of expansion) which is not that important, the important thing is the targets after that, Abdju and Waset. Neither of them likes you, and they really dislike your wife and ally, but most importantly both provinces provides lots of legitimacy thanks to the density of special landmark outposts, in Abdju every region has one and in Waset all but one has one, so securing those two provinces is a very good idea (Waset also gives you access to the better Pharaoh hats so that is also a reason to secure that territory).

So in short get legitimacy and if there is only one potential troublemaker name them heir for the time being so they can't start anything, if there are two potential troublemakers naming one will atleast prevent that one from starting anything.