Lenin once argued that revolutions are a product of two conditions. Firstly, the "lower classes" must be unwilling to carry on living in the old way but secondly, that the "upper classes" cannot carry on ruling in the old way. Any great crisis involves splits at the top of society which in turn open up space for revolts from below.
The repressive forces of the state machine, its police, army, even intelligence services, can become paralysed as they are unsure which section to obey, or which will win out. The normal operation of the mass media can break down, allowing space for other, more radical, voices.
Crises that begin at the top can in this way unleash much more radical forces from below, but there is also the danger that movements from below are, or become, tied to the maneoverings of a section of the exploiters which in turn can become bound up with inter-imperialist rivalries.
The weight of these different elements has to be assessed in each upheaval. The protests over the past few months in Thailand and Venezuela have been led by sections of the ruling class.
By contrast the protests that erupted last summer in Turkey and Brazil were genuine mass movements, where protests over rising bus fares in Brazil and opposition to development plans for Istanbul's Gezi park, became a lightning rod for wider discontent. But other forces, the right wing opposition in Brazil and nationalists sympathetic to the Turkish military, then attempted to push the movements along different paths.
In Ukraine the initial protests over Yanokovitch's decision to abandon an agreement with the EU turned into something much wider in the face of state brutality. But the movement appears to have remained heavily influenced by a section of powerful oligarchs. As bloody clashes developed, the fascists proved the best organised part.
The weight of independent initiative from below and from workers has been weak, so far at least, compared to ruling class maneuvers and the role of reactionary forces. This can change but its absence raises a number of dangers.
Firstly it increases the risk that ethnic politics will trump class unity, with the struggle increasingly posed as between a EU-oriented western Ukraine versus a Russia-oriented east.
In turn this raises a second danger: the conflict will become entangled with the imperialist competition between the EU and US on the one hand and Russia on the other.
The Crimean peninsula is the potential flashpoint. A few days after Yanokovitch was overthrown, pro-Russia protests in Crimea where followed by an armed group seizing the regional parliament, with MPs calling a referendum on the region's relationship to Ukraine on the same day, 25 May, as snap president elections are planned across the Ukraine.
Reports circulated of Russia military maneuvers. The shadow of the wars in Yugoslavia hangs over the Ukraine -- though Russia is a more confident power today than in the mid 1990s, raising the stakes even higher.
The outcome of events is unpredictable. But they underline another key element that Lenin insisted on -- the outcome of any uprising is ultimately decided by questions of political leadership. Ukraine serves as a warning: in the absence of a strong organised left then reactionary forces can end up advancing the cause of barbarism, not socialism.