Tensions are continuing to rise between Ukraine and Russia over the Crimean Peninsula border, after a weekend incident in which alleged Ukrainian infiltrators killed a pair of Russians, including a soldier and an FSB security service member. The Ukrainian government denied not only that they had anything to do with it, but that the incident happened at all.

Amid reports of Ukrainian military buildup along the border, Russia has announced the deployment of an S-400 air defense system into the Crimean Peninsula. The advanced missile system is designed to prevent enemy combat aircraft access to the surrounding airspace.

The S-400 is one of Russia's signature top-end technological assets, and the deployments are usually seen as proof Russia "means business" in a given situation.