I am just going to copy my response to u/sean951 here since yall are kind of asking similar questions:

When strategy and technology is similar between warring factions, the stronger actor is much more likely to win. This becomes increasingly difficult when the strategies differ (conventional strategy v. unconventional strategy) and when technology gaps widen beyond a certain point.

Check out this paper, in PDF form, on the subject.

Guns make it easier, but back then if an area rebelled, they'd send in the army and kill anything that resisted, burn the fields, arrest family and friends, torture etc. And they would do this for a generation or two until it integrated.

Yes, if strong actors are willing to commit that level of time and resources to a situation, they can win. But what makes weaker actors gain the upperhand in these kinds of conflicts is that their will to fight is much higher and the stakes for them losing are also higher. Strong actors will usually cut their losses at the risk of Pyrrhic victories against small-time actors with limited economic gains (given the cost of maintaining such low-level combat over extended periods of time).

Not all old empires faced these kinds of conflicts to the extent we do now. Not all old empires encountered and conquered others with such vast technological and strategic asymmetries as we do now. They did happen, but the technology gap had a much different effect on those conflicts than they do now.

Today, people get rightly upset if you accidentally kill civilians.

Hence why it is increasingly difficult to wage war on weak actors. They can hide behind civilians with relative ease, particularly if their arms are predominately small arms. It is far harder to hide behind civilians when your forces have tanks and aircraft.

A lot of military strategy is counter-intuitive, at least on the surface. Weak actors having a better chance today at defeating strong actors? Sounds kinda strange, but we see this time and time again. The closer you move to the modern day, you see fewer and fewer successful campaigns waged by strong actors (US, France, Britain, etc.) domestically or abroad. The most recent one I can think of is the end of the Troubles in regards to Northern Ireland. But even then, violence has continued, and politically, the situation is still tense.

Some point to Russian action in Chechnya as an example of how it is futile to stand up to major governments, but in reality, the Chechens won. The Chechens got essentially their own Putin to run their region. Putin knew the only way to end the conflict swiftly was to give the Chechens essentially what they wanted without actually granting them full independence.