(CNN) -- It's a conflict culled from the suspicion-filled pages of the Cold War.
Russia's upper house of Parliament gives the green light for the incursion of military forces into Ukraine's volatile Crimea region.
President Vladimir Putin justifies the move as a necessary step to the protection of Russian citizens and military personnel in southern Crimea.
Ukraine's new government calls out what it calls Russia's threat on its sovereignty.
U.S. President Barack Obama warns of the "costs" Russia faces for its actions.
With Ukraine possibly teetering on the brink of war, here are three things you need to know about the conflict.
1. What is the Black Sea Fleet?
A justification for Russian might in the region is its claim of the need protect the Black Sea Fleet in the Ukraine.
Based in Sevastopol, the force is the smallest of the Russian navy's four fleets -- which once included some 18 submarines, two cruisers, 30 destroyers and frigates and around 100 smaller combat ships, according to Jane's International Defense Review. Many vessels are aging and in need of maintenance.
The Russian navy has had a base in Sevastopol for 230 years. The ships and subs are based just north of Turkey and can reach the Mediterranean.
The fleet has been a point of contention since 1954, when the former Soviet Union transferred the Crimea, including Sevastapol, to Ukraine, according to Jane's. In 2010, the two countries reached an agreement to permit the fleet to stay in Sevastopol until 2042.
Under the deal, Ukraine received a 30% discount on the cost of natural gas supplies from Russia, potentially saving Ukraine up to between $40 billion over a10-year period.
The fleet has allowed Moscow to exert its influence in the Eastern Mediterranean, Balkans and Middle East, according to Christian Le Miere, an expert at the International Institute for Strategic Studies.
"The importance of the Mediterranean to Russia was highlighted in 2013, when, amid the civil war in Syria, Moscow declared the creation of a 'permanent task force' for the sea and bolstered its presence to 10 vessels," he wrote. "In August 2008, it was the Black Sea Fleet that provided the 13 vessels used to defeat the small Georgian navy and land troops in Abkhazia and Poti during the brief war."
2. How close is Russia to sending more troops?
According to the Russian state news agency Itar-Tass, the country's constitution requires that the Federation Council must consider and debate the use armed forces outside Russia following a request from the president. The request should contain "sound reasons" for the use of the military.
The chair of the council then sends the request to the upper house's committee on defense and security and the committee on international relations.
After the council considers the request, the president and the prime minister are invited to attend a council meeting, according to Itar-Tass. The meeting opens with a report delivered by the president or a representative.
Then, the conclusions of the council committees on defense and security and international relations are read.
A final decision on allowing the use of the armed forces outside the country must be adopted by a majority of the council members and and made formal in a resolution of the upper house, according to the news agency. The document is forwarded to the president within two days of its adoption.
3. Is there a precedent for what might unfold?
In 2008, Russia's incursion into the former Soviet republic of Georgia followed the launch of a Georgian campaign against the Russian-backed separatist territory of South Ossetia.
Russian tanks, troops and armored vehicles poured into South Ossetia and another breakaway Georgian territory, Abkhazia, advancing into Georgian cities across the administrative borders within those regions.
The two sides blamed each other for starting the conflict, as well as for a wide variety of offenses leading up to and during the fighting, including ethnic cleansing.
Russia and Georgia signed a French-brokered, six-point cease-fire agreement that allowed Russian forces to establish a buffer zone inside Georgia within a few kilometers of South Ossetia.