After days of simmering tension, the world's worst fears came true when Russian President Vladimir Putin declared war on Ukraine in a televised address on Thursday. He said the military action announced by him will seek to "demilitarize" Ukraine and came in response to threats from Ukraine.

Moments after his announcement, large explosions were heard in different parts of Ukraine. Artillery attacks have also been reported at Ukraine's borders.

Responding to the developments, Ukraine Foreign Minister Dmytro Kuleba said, "Ukraine will defend itself and will win." Ukraine also announced the imposition of martial law, which means strict restrictions including bans on meetings, movements and political parties.

Meanwhile, US President Joe Biden condemned Putin's order of invasion and said he would be coordinating with NATO allies to respond to Russia's aggression.

Amid these major developments, here's everything you need to know about the Ukraine-Russia crisis.

Timeline So Far

Nov 10, 2021: US reports unusual Russian troop movements near the Ukrainian border. Moscow denies, demands legal guarantee that Ukraine will never join NATO.
Dec 7, 2021: US threatens Russia with "strong economic and other measures" if Ukraine invaded
Jan 17, 2022: Russian troops begin arriving in ex-Soviet Belarus for military drills
Jan 19, 2022: US announces extra $200 million in security aid to Ukraine
Jan 24, 2022: NATO puts troops on standby, sends ships and fighter jets to eastern Europe
Jan 25, 2022: Moscow begins military exercises near Ukraine and in Moscow-annexed Crimea
Jan 26, 2022: US refuses to shut NATO door on Ukraine
Feb 2, 2022: US sends 3,000 troops to eastern Europe
Feb 10, 2022: Russia and Belarus begin 10 days of military manoeuvres
Feb 17, 2022: Shellfire intensifies along Russia-Ukraine frontline
Feb 22, 2022: Russia recognises independence of two separatist regions in eastern Ukraine. Putin deploys troops in these areas. EU, US, UK impose sanctions
Feb 24, 2022: Russia declares war on Ukraine. Putin warns outsiders not to interfere. Explosions heard in Ukraine.

Why Has Putin Declared War?

In an article published in July last year, Vladimir Putin described Russians and Ukrainians as “one nation” declaring the collapse of the Soviet Union (USSR) in December 1991 as the “disintegration of historical Russia”. Putin believes that Ukraine's leaders are running an “anti-Russian project”.

Russia under Vladimir Putin disapproves of the expansion of the US-dominated North Atlantic Treaty Organization (NATO) in its neighbourhood, particularly Ukraine's steps towards it. Russia feels threatened by the rapid expansion of NATO since the late 1990s in the region. NATO and the former USSR were engaged in what was called the Cold War for about 45 years in the previous century.

Ukraine is the largest country in Europe if one takes out Russia, which is spread over Asia as well. In 2014, Russia annexed Crimea, a peninsula of Ukraine.

Role of US, West

While Russia has wanted the US and its allies to stop pushing Ukraine, part of the former Soviet Union, into NATO, the US and NATO believe that countries should be free to join any alliance. The NATO countries have been lending support to Volodymyr Oleksandrovych Zelenskyy-led Ukraine against Russian aggression.

Ukraine has, thus, become a cold war-type battleground for the power struggle between US and Russia.

After Russia recognised two separatist regions of Ukraine as independent, the US, UK and European Union among others announced sanctions on Russia.

"We're imposing sanctions on 2 Russian financial institutions. Russia can no longer raise money from the West or no longer trade in Western markets. We'll also impose sanctions on Russian elites and their families," Joe Biden had said.

Separatist Regions

The Donbas part of Ukraine houses two Russia-backed separatist regions Donetsk and Luhansk, which were recognised as independent by Russia earlier this week.

Ukraine regards the Donetsk People's Republic and Luhansk People's Republic as terrorist organisations and the regions as temporarily occupied territories as a result of Russian military intervention.

On the other hand, Russian President Putin had alleged that Ukraine plans to unleash a war against Russians in Donbas and is acting on the orders from the West. He went on to move troops to the regions, before declaring full-blown war on Ukraine on Thursday.

How World Reacted

US President Joe Biden said, "I'll be monitoring the situation from White House this evening and get regular updates from my national security team. Tomorrow, I'll meet with my G7 counterparts in the morning... We'll coordinate with our NATO allies."

He added, "Russia alone is responsible for the death and destruction this attack will bring. The US and its allies will respond in a united and decisive way. The world will hold Russia accountable."

Meanwhile, Pakistan PM Imran Khan, who is currently on his maiden visit to Russia, reportedly said, “What a time I have come - so much excitement."

German Chancellor Olaf Scholz said Russia's attack on Ukraine was a "dark day for Europe".

UK Prime Minister Boris Johnson said he "hopes Ukraine can resist Russia".

NATO chief Jens Stoltenberg denounced Russia’s “reckless and unprovoked attack” on Ukraine and said it has put “countless” lives in jeopardy.

Meanwhile, India's Ministry of External Affairs is closely monitoring the situation and focusing on the safety of Indians in Ukraine.