Ukraine crisis: Why now? What's the history? What are Putin's motivations?

Just six months ago, Vladimir Putin opened a 7,000-word manifesto on the historical ties between Ukraine and Russia with the claim that Russians and Ukrainians were “one people”, part of “a single whole”.

But to the "common misfortune and tragedy" of both countries, Putin said, in recent years "a wall has emerged" between Russia and Ukraine.

There are those who see Putin's comments as part of the explanation for the massing of tens of thousands of troops on its borders with Ukraine, a move that has prompted Western officials to issue increasingly alarming warnings that Russia is gearing up for an invasion.

Defence Minister Sergey Lavrov warned Wednesday that Russia would take "necessary countermeasures" if the West "continues on its aggressive course," but Moscow has denied that it is planning to invade.

Russia considers Ukraine becoming more integrated with NATO as a threat to its national security and has demanded that the military alliance does not expand eastward to Ukraine or other former Soviet states. It has also demanded that NATO roll back its military deployments in Central and Eastern Europe.

And while it’s anyone’s guess as to what will happen next, analysts say Russia’s military buildup is larger this time, with fears ranging from Putin escalating the war in eastern Ukraine to launching a full-scale invasion.

Russia has amassed more than 100,000 troops near the Ukrainian border, US officials say, including moving troops to neighbouring Belarus for military exercises.

This has included sending more equipment to separatists in eastern Ukraine, according to the Ukrainian defence intelligence.

It said Russia had this month sent “7,000 tons of fuel, several tanks and self-propelled artillery systems, other weapons and