A screenshot from Espreso TV's live feed from Kiev on Feb. 18. (Espreso TV)

A growing number of reports from Kiev suggest that some of Ukraine's protesters, after three months in Independence Square and several days enduring a brutal government crackdown, may have upgraded from molotov cocktails to firearms. Those reports are not conclusive, and they do not portray the protesters as picking up guns en masse. But the reports are coming in with enough frequency that it's worth giving some credence to the possibility that some protesters, after being shot at for days by police, are now shooting back.

You could hardly blame the besieged protesters if they wanted to fight firepower with firepower. Still, if this were to become a trend and protesters were to increasingly bear and use arms to defend against security forces, that could actually spell doom for their movement. It could be the best thing to happen to Ukrainian President Viktor Yanukovych since clashes reignited earlier this week.

When civil resistance movements popular uprisings adopt small arms use, they become much more likely to fail and the government becomes more likely to survive intact. That was a conclusion I reached, anyway, as part of my graduate research on government crackdowns against popular democratic movements. I surveyed 20 attempted uprisings over the past century and found overwhelming evidence that when protesters take up arms in large numbers they make their movement far likelier to fail. Within those cases, the chance that a popular uprising would "outlast" the government and ultimately secure its goals was cut almost in half if the protesters took up arms.

There a few reasons for this. One of them is that shooting back makes the conflict less about politics and more about simple force, a contest of strength that disorganized civilians are often bound to lose. Probably the biggest is the military: when protesters start shooting back at security services, it makes the military much more likely to intervene against them. And if the military intervenes against protesters, that usually ends things pretty quickly.

It's really, really difficult to overstate the importance of the military in moments of mass popular protest or uprising: ultimately, they have the monopoly on force, and they can decide the outcome if they want to. Militaries often try to stay out of this stuff, as appears to be happening now in Ukraine, where Yanukovych recently fired his army chief, which some speculate could be over disagreements about whether the military should intervene. But when protesters and police are shooting one another in the streets, the military becomes more likely to step in.

Partly this is about the generals feeling compelled to restore physical security. Partly, the protesters are legitimizing the use of deadly force, and eroding the taboo against open shooting, making it much easier for generals to ask their troops to open fire. And once that happens, once the protest-and-crackdown dynamic becomes a shooting match between protesters and troops, it's pretty much over because the military will outgun them almost every time. That's not to blame them the protesters, of course, but just to observe how this stuff happens.

When protesters take up arms, it also appears to often have an effect whereby the military doesn't just become more likely to intervene – the generals become more loyal to the government. There's a rally-around-the-flag effect. And the military becomes much less likely to lean on the government to make concessions.

Oftentimes, popular uprisings ultimately succeed because the military sides with them, explicitly or implicitly. We saw this in Romania in 1989, for example, when the military largely refused orders to put down protests, helping to doom leader Nicolae Ceaușescu. And we saw it in Egypt in 2011, when the military parked tanks around protesters to protect them from security forces, and shortly after forced out President Hosni Mubarak. But that becomes much less likely if protesters start using small arms. Militaries are just much less comfortable aiding a protest movement that shoots back.

To a lesser extent, when protesters take up arms it appears to ultimately reduce popular support for their movement. The hardcore coterie of activists might see the need for armed resistance, but most civilians tend to value their physical security highly, and are more likely to support the government if they believe the opposition is sowing violence. To shoot back is to risk alienating your own "quiet" supporters.

But the military is the big factor here. The protesters need the military to some extent, even if it's only for troops to stay in their barracks. If protesters take up arms, they're much less likely to have military support or even acquiescence. Worse, the military becomes more likely to intervene against them – and the rest of the population becomes more likely to support that. None of this is good for anyone, except perhaps Yanukovych.