On the one hand, we have air strikes, drones, artillery and what not - easy kills from a distance.  But on the other hand, pretty much anyone can put together a rather nasty bomb, and an AK-47 can turn just about anyone into a soldier.  

That wasn't the case in the medieval and early modern worlds, when weapons were hard to use effectively without training and experience.  It was harder for a popular resistance to be effective.

Another key difference in the modern world is the ideology of nationalism.  On average, ordinary people seem to care more about who their rulers are than in the past, and are much less likely to tolerate a foreign conquering power.