The S-400 can hit targets well over 150 miles from its launch site when paired with the appropriate radar array and is billed as one of Russia’s most advanced surface-to-air defense systems. The Crimean Peninsula is currently home to the older S-300 Russian surface-to-air missiles that will probably be replaced by the arrival of the newer S-400s.

Between Aug. 6 and Aug. 8, the Russian Federal Security Service, or FSB, said it arrested Ukrainian saboteurs who had sneaked over the border and also recovered a large cache of explosives in the town of Armyansk. During the arrest, one FSB agent was killed in an exchange of gunfire with the Ukrainian agents, according to a statement on the FSB’s website. The FSB also claimed to have repelled Ukrainian special forces attempting to infiltrate Crimea and in a subsequent cross-border shelling, one Russian soldier was apparently killed.

Eyewitness accounts to the incident have been spotty, and social media reports indicate there were Internet outages in northern Crimea during the FSB operation. The Ukrainian government has denied that it condoned any type of border incursion or that it had shelled Russian forces from its territory.

In 2014, Russia deployed troops to Crimea following the ousting of former Ukrainian president Viktor Yanukovych. A subsequent and much-disputed referendum allowed Russia to officially seize the region, though the annexation has been deemed illegitimate by a number of countries, including Ukraine and the United States.

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Since the FSB announced the arrest of the Ukrainian attackers, Russian President Vladimir Putin has accused the Ukrainian government in Kiev of “practicing terror,” and on Thursday, parts of Russia’s Black Sea Fleet began a series of exercises off the coast of the disputed peninsula. According to the Russian Defense Ministry’s website and U.S. defense officials, the exercises had been planned for sometime.

The month of August usually coincides with an uptick in Russian military training activity. Videos on social media have shown significant amounts of armored vehicles, air defense systems and anti-ship missiles flowing into Crimea from Russia on rail cars and on highways. Ukrainian forces have also reported unusual activity along their front lines in eastern Ukraine where they have been fighting Russian-backed separatists since April 2014. The surge in activity has put Ukrainian forces on high alert for a possible offensive, though since Ukrainian lines have remained mostly unchanged since early 2015, rumors of breakthroughs and large-scale attacks have become an almost monthly occurrence.

The combination of military maneuvers, saber rattling and sharp comments from the Kremlin have prompted Western nations to call for de-escalation of tensions in the region, and Ukraine requested Friday that the Organization for Security and Cooperation in Europe place more monitors on the Crimean border.

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Since Russia’s annexation of Crimea, the Russian navy has significantly bolstered parts of the Black Sea Fleet stationed in Sevastopol, Crimea, and shuffled units from its southern military district onto the peninsula, placing contingents of artillery, airborne infantry, aircraft and surface-to-air batteries there. When Russia took Crimea — historically one of the most strategically important parts of Ukraine — it absorbed thousands of Ukrainian forces that were subsequently assimilated into the Russian military.