What happened

Russian tech stock star Yandex (YNDX) fell 10.2% through 3:15 p.m. ET on Monday -- and you can probably blame Russian President Vladimir Putin for that.

As the English-language The Moscow Times said this morning, currently both the Russian ruble and Russian stocks are "in the red as talk of military confrontation spooks investors." 

World map with Russia under a magnifying glass.

Image source: Getty Images.

So what

For weeks, diplomats have been warning that the Russian Federation is massing troops on Ukraine's eastern border -- more than 92,000 at last count. For weeks, investors have been able to shrug off those worries on the assumption this was just a repeat of what happened in 2018, when Russia built up its troop strength on the border, only to draw it back down again -- but this time could be different.  

For one thing, the number of soldiers being gathered together is greater than three years ago. For another thing, Ukraine is warning that this could be the prelude to a large-scale invasion of Russia's neighbor as early as January or February, reports the Military Times today.  

Now what

Of course, there's still the question of why Russia's (still potential) decision to start a land war in Eurasia is necessarily bad news for Yandex. Russia's military, after all, is nearly five times the size of Ukraine's, and is backed by a Russian military budget 12 times as large as Ukraine's. Should Russia invade Ukraine again, the conflict would not likely spill over into any significant portion of Russian territory, and -- absent assistance from NATO -- Russia would probably prevail.  

What worries investors more, I suspect, is the potential that NATO and other countries would respond not with military aid for Ukraine, but with tightened economic sanctions against Russia -- up to and perhaps including banning Russian stocks (such as Yandex) from trading on U.S. stock exchanges.

That's a worry worth considering. I suspect it's uppermost in investors' minds as they sell off Yandex stock today.