Fifteen years ago, the G8An organisation made up of the world's largest economies. It became the G7 in 2014 when Russia was suspended for invading and annexing Crimea. met in Germany. Vladimir Putin, nearing the end of his second term as Russian president, was there. He discussed economic coordination and defence with US president George W Bush.

On Monday, an older Putin threw down the gauntlet to the West. He declared that he would recognise the independence of two breakaway statesTwo regions in eastern Ukraine, Donetsk and Luhansk, declared independence in 2014, with Russian backing. Since then 14,000 people have died fighting over these regions. in eastern Ukraine and ordered troops into the country.

How did so much change? Some think Putin has changed. He was seen as a rational leader, bringing the skills he learnt in the KGB to defeat the oligarchs in Russia.

In the last week he has seemed more like a power-crazed tsarTitle given to an emperor of Russia before the revolution of 1917. The word was originally used for the Bulgarian monarchs in the 10th Century, but can also be used to refer to anyone with absolute power..

His actions in Ukraine also strike at the heart of NATO. Its self-declared aim is to guarantee the freedom and security of its members. Ukraine is not a member, but it has long been regarded as a NATO ally and a future member. 

NATO powers have previously avoided direct confrontation with Russia, instead imposing economic sanctions when necessary. Now they have to decide whether to respond with force.

Russia is the world's most heavily-armed nuclear state. Its nuclear arsenal is bigger than that of all of NATO's members combined. Putin has brought the world closer to nuclear war than it has been for decades.

Some have compared this to the Cuban Missile Crisis of 1962. Cuba had been ruled by fascist dictator Fulgencio BatistaA dictator who seized power in Cuba in a coup in 1952, having previously served as president. He is thought to have murdered around 20,000 Cubans., a US ally, but in 1958 a communist group led by Fidel CastroA communist dictator who governed Cuba from 1959 to 2008, when he handed power over to his brother, Raul. overthrew him. 

In 1961, the CIA sponsored an attempted coup by anti-communist Cuban exiles, known as the Bay of PigsIn 1961, 1,500 Cubans who had fled Castro's rule tried to seize power on the island, with covert US backing. Their attempt failed and it caused a political crisis in the USA.. The coup was crushed. An outraged Castro cut all ties with the USA, turning to the USSR.

The USSR responded by stationing missiles in Cuba. The USA demanded they be removed. Some of its top generals wanted to declare war.

In the end, the two sides came to an agreement. But it was the closest the world has ever come to nuclear war.

In 1962, the USSR was supplying missiles to a friendly government. In contrast, Putin's allies in eastern Ukraine only control around half the territory he has now recognised as theirs.

Russian soldiers could end up bogged down in a guerrilla war. There is a risk that this could be a rerun of Afghanistan - the 13-year-long war that brought down the Soviet Union.

Yes: The world's nuclear superpowers are once again at loggerheads as their spheres of influence clash. Even if a solution is found, there is likely no way back to the warm relations of the 2000s.

No: In truth, Russian soldiers have been operating in eastern Ukraine for years. There is little NATO can do to stop them. This is more like Crimea in 2014: the world will gasp, then move on.

Or... In 1962, the USSR was a self-confident state that believed it owned the future. Today, Russia is a second-rate power with a corrupt government and unhappy people. It is acting out of weakness, not strength.