We just spent 20 years in Afghanistan proving how effective and potent a modern military is against guerilla tactics. I think it's become pretty clear that if you're not willing to ignore the generally accepted rules of engagement, they will just outlast you.
Not to mention the Vietnam war or any of the other wars we've been involved with in the Middle East. Not to mention drug cartels in Mexico and Latin/South America.
I'm not sure how anyone can argue that a population with weapons can't hold off a tyrannical government. It is the same reason Marx said that under no means should we disarm the populous. I think people often forget that a government can't just kill all its population. Not only can you not rule over the dead but you also are going to have a hard time interacting in global trade (thanks globalism, but this even held true in the 1700's). It is kinda odd that this sentiment also often comes out of a country where some farmers fought off the country with the biggest navy and had an empire so great that the sun never set on it. Civil wars are extremely costly to countries. Also I'm not sure why everyone thinks that people from the military (including generals and entire battalions) wouldn't defect. That's also pretty common when looking at history.
Which has also been common throughout history. Same thing happened during the US civil war, Europe got involved. Of course other countries would get involved. Why would it be different this time?