Let us say, for the sake of argument, that you live in the middle of the fucking desert a couple of thousand years ago.
You are (by accident of birth) quite high in your society's hierarchy - but a lot of things that we take for granted these days simply don't exist in the middle of this fucking desert.
There's no police force.
There's no farm subsidies (though there are farmers, it's a hard life being a farmer in the middle of a fucking desert).
The last couple of years have been tough - there's virtually no water (though the people in the next country have a number of rivers...). As a consequence, many of your people are starving.
There may or may not be such a thing as conscription, but keeping morale up in your army is damn hard. You can barely feed them, FFS.
There is precious little that would be recognised as a legal system two thousand years hence.
There's no international aid - it's every man for himself.
There's little education - those who aren't from a wealthy background (which 99% of your countrymen aren't) don't necessarily understand exactly how bad things are or how best to improve them.
What your people need to survive is some better farmland. And you know exactly what you need to get it - water. Easiest way to get this is to re-settle as many people as you can next to the neighbour's river. Your neighbour is likely to object, however, so you'll have to take it by force. How on Earth do persuade thousands of people to take up arms and invade your neighbour?
Well, like most leaders you're fundamentally a politician. Two thousand years from now you'd spin your people some line about how this neighbouring country has big scary weapons, but that doesn't work so well here because nuclear weapons aren't going to be invented for some time.
The most powerful thing you have available is your own local myths and legends - which include legends about how the world was created by an all-powerful being. Most of your people believe pretty firmly in them. So you spin them a line about how this all-powerful being has promised them the world - on condition they take on any "non-believers". And by an amazing coincidence, there's a whole bunch of non-believers in the next country.