Explained: What prompted Putin's invasion of Ukraine, Russia's global defiance

Russia launched a 'military operation' against Ukraine - effectively an invasion - early Thursday morning, triggering global fears of a major multi-national armed conflict. Russian President Vladimir Putin announced the attack in a televised address, which follows his declaration that a 2015 peace deal agreed to stop fighting over breakaway regions of Donetsk and Luhansk - is invalid.

What followed was reports of explosions and missile strikes at key Ukraine cities, including capital Kyiv and in the rebel regions of Donetsk and Luhansk. Russia, according to western intel, has amassed well over 150,000 troops and significant military equipment along the Ukraine border.

So far, Russia has claimed the destruction of airbases and air defences, while Ukrainian military says it has shot down six Russian aircraft in Luhansk.

At least eight people have been killed and nine wounded by Russian shelling, an advisor to Ukraine's Minister of Internal Affairs was quoted by Reuters.

Russia's actions have been condemned by almost all major nations, including the United States, the European Union, the United Kingdom, and Canada. 

UN chief Antonio Guterres has urged Putin to stop the war ‘in the name of humanity’, warning of devastating consequences for Ukraine and the world.

Why has Russia attacked Ukraine?

Russia believes Ukraine is moving closer and closer to the West via both the NATO, or North Atlantic Treaty Alliance, and the European Union. 

Ukraine is not a member of NATO but has cooperated with the alliance and frequently expressed its intention to sign up.

Putin, however, is aware Ukraine joining NATO significantly increases the difficulty of bringing Donetsk and Luhansk within his control. He has also frequently accused Ukraine of being a 'puppet' in the hands of the West.

The removal of pro-Russian Viktor Yanukovych as President of Ukraine - he was voted out by the Ukrainian parliament in February 2014 - triggered Russia's annexation of the strategically valuable Crimea peninsula eight years ago.

A former member of the Soviet Republic, Ukraine still has deep social, historical, cultural, and political ties to Russia, but relations have been on a knife's edge since the 2014 invasion.

It gained independence in 1991 but has struggled economically since.

Fearing it could be surrounded by what it sees as hostile forces, the Kremlin has demanded guarantees NATO will not accept Ukraine, or other former members of the Soviet Republic. Western countries, and NATO, have dismissed such fears, but Putin is clearly not buying their statements.

Russia's defiance in the face of sanctions

The US, the European Union, the UK and other countries have all slapped sanctions on Russia, targeting its banks (to cut funding to operations in rebel regions) and the lawmakers who approved the use of force.

Putin, however, seems unconcerned.

Earlier this week Russian Foreign Minister Sergei Lavrov brushed off the threat of sanctions, saying, "We're used to it. We know that sanctions will be imposed anyway, in any case. With or without reason."

"They are already threatening us with all manner of sanctions or, as they say now, 'the mother of all sanctions'," he said.

Putin is also possibly secure in the knowledge that the US and other western nations will think twice (and maybe thrice) before deploying troops themselves, and risk turning this conflict into a potentially global one.

The threat of a prolonged military operation is something Putin can sustain almost indefinitely, if he is willing to pay the cost of keeping a large number of troops and equipment in the field, according to two retired US military commanders quoted in a report by Bloomberg.