Seldom in the field of human conflict did so much hang on the whims of one man. Is Russian President Vladimir Putin about to invade Ukraine, as the massing Russian troops on its borders suggest? Or is he bluffing, to extort concessions from his neighbour and the West? No one can be sure of Mr Putin's intentions. Even his own foreign minister seems to be kept guessing. But, if fighting is about to break out, the world needs to understand the stakes.

Perhaps Mr Putin is planning a full-scale invasion, with Russian forces thrusting deep into Ukraine to seize the capital, Kiev, and overthrow the government. Or he may seek to annex more territory in eastern Ukraine, carving out a corridor linking Russia with Crimea, the Ukrainian peninsula Mr Putin grabbed in 2014. Then again, he may want a small war, in which Russia "saves" Kremlin-backed separatists in Donbas, an eastern region of Ukraine, from supposed Ukrainian atrocities - and, at the same time, degrades Ukraine's armed forces.