After a standoff spanning several weeks, Russian troops have reportedly begun invading Ukraine. Tensions are now higher than ever, and a war is underway. 

Airlines have already been canceling flights to the Ukrainian capital of Kyiv due to concerns of passenger safety, especially considering the crash of Malaysia Airlines flight 17, which was shot down by Russian rebels in Ukraine. Now that the fighting has begun, civil airspace over the whole country of Ukraine has been closed.

Popular flight tracking application FlightRadar24 tweeted, "Per NOTAM, flights by civil aircraft are now restricted inside Ukraine." A snapshot of ADS-B flight traffic in the area reflects this:

According to BNO News, Ukrainians have also begun taking initiatives to prepare for the worst. Many farmers have driven tractors onto the runways of nearby airports in order to prevent the Russians from landing and setting up base.

This is a rapidly-developing story.