It is a bit of an axiom to state that geography dictates history. Leaders through time immemorial have played the cards that were dealt them in terms of their land's terrain, mountains, rivers and climate. In today's world, with technology essentially shrinking distance, we may be a little inclined to discount the true influence of geography on world matters. The recent Atlantic piece by Tim Marshall entitled, Russia and the Curse of Geography, does an excellent job of underlining its critical importance - notwithstanding technology.
The European Plain, directly to Russia's west, has throughout history been a challenge for Russia. With no significant geographical barriers, this has been the path of many invaders into the motherland. The Poles came across the European Plain in 1605, followed by the Swedes in 1707, the French under Napoleon in 1812, and the Germans twice, in both world wars, in 1914 and 1941. The plains would also explain why Russia has repeatedly invaded Poland - occupying Poland allows for a defence of the plains at their narrowest point (300 miles).
The other key challenge to Russia is access to warm water ports. Without those, Russia is limited to the world's busiest sea-lanes only during warmer weather. Their northern ports are in the Arctic and their major port on the Pacific Ocean, Vladivostok, is enclosed by the Sea of Japan, and under largely Japanese control.
These two key strategic factors, Russia's vulnerability on land and its lack of access to warm-water ports, came together in Ukraine in 2014. When protests in Ukraine brought down the pro-Russia government and a new, more pro-Western government came to power, Putin did what Russian leaders have done for centuries. He chose his own kind of attack as defense, annexing Crimea to ensure Russia’s access to its only proper warm-water port, and moving to prevent NATO from creeping even closer to Russia’s border.
The article goes on to discuss similar rationale regarding Syria. Click the link for a fascinating read and reminder of the power of geography.