Russian involvement in the Syrian civil war

Between 1955 and 1958, Syria received about $294 million from the Soviet Union for military "assistance".

The Syrian Revolution of February 1966 gave the Soviet Union the opportunity to further "support" Syria.

In 1971, under an agreement with the Syrian Ba'athist government's President Hafez al-Assad, the Soviet Union was allowed to open its naval military base in Tartus, giving the Soviet Union a military presence in the Middle East.

The Syrian Civil War is an ongoing international armed conflict taking place in Syria.

The unrest began in the early spring of 2011 within the context of Arab Spring protests, with nationwide protests against President Bashar al-Assad's government, whose forces responded with violent crackdowns.

The civil uprising phase created the platform for the emergence of militant opposition movements and massive defections from the Syrian Army, which gradually transformed the conflict from a civil uprising to an armed rebellion, and later civil war.

On 22 September 2014 America shows up, the United States intervened in the Syrian civil war to fight the Islamic State as part of Operation Inherent Resolve in the international war against the Islamic State.

What I have written above is the facts of how Syria became a battle ground. The sources and more are listed in Wikipedia by using the search terms, "Russian involvement in Syria"