KYIV — Russian troops launched a wide-ranging attack on Ukraine on Thursday, with explosions heard before dawn in the capital Kyiv and other cities.

In a televised address as the attack began, Putin warned other countries that any attempt to interfere would “lead to consequences you have never seen in history.”

Ukrainians started fleeing some cities, and the Russian military claimed to have incapacitated all of Ukraine’s air defenses and air bases within hours.

World leaders decried the start of an invasion that could cause massive casualties, topple Ukraine’s democratically elected government and threaten the post-Cold War balance on the continent.

U.S. President Joe Biden declared that the world will “hold Russia accountable.” NATO Secretary-General Jens Stoltenberg condemned Russia’s action as a violation of international law and a threat to European security.

Ukraine’s foreign ministry said Russia’s intent was to destroy the state of Ukraine, a Westward-looking democracy intent on moving out of Moscow’s orbit.

Here are the things to know about the conflict over Ukraine and the security crisis in Eastern Europe:

PUTIN MAKES HIS MOVE

Putin said the military operation was needed to protect civilians in eastern Ukraine — a claim the U.S. had predicted he would falsely make to justify an invasion.

Putin accused the U.S. and its allies of ignoring Russia’s demands to block Ukraine from ever joining NATO and offer Moscow security guarantees.

Putin said Russia does not intend to occupy Ukraine but will “demilitarize” it. Soon after his address, explosions were heard in the cities of Kyiv, Kharkiv and Odesa. Russia said it was attacking military targets.

He urged Ukrainian servicemen to “immediately put down arms and go home.”

Ukraine’s border guard agency said the Russian military has attacked from neighboring Belarus, unleashing a barrage of artillery. The agency said Ukrainian border guards fired back, adding that there was no immediate report of casualties. Russian troops have been in Belarus for military drills.

THE WEST REACTS QUICKLY

Biden, Stoltenberg and other world leaders quickly condemned Russia’s attack as unprovoked and unjustified.

Putin “has chosen a premeditated war that will bring a catastrophic loss of life and human suffering,” Biden said in a statement.

British Prime Minister Boris Johnson says Western allies will not stand by as Russia attacks Ukraine, and told Zelenskyy in an early morning call that he was appalled by events.

Japanese Prime Minister Fumio Kishida said: “This Russian invasion stands to put at risk the basic principle of international order that forbids one-sided action of force in an attempt to change the status quo.”

UKRAINE’S PRESIDENT URGES CALM

Residents of Ukraine’s capital, Kyiv, could be heard shouting in the streets when the first explosions sounded. But some kind of normalcy quickly returned, with cars circulating in the streets in the early morning commute.

President Volodymyr Zelenskyy issued a video statement declaring martial law. He told Ukrainians that the United States was gathering international support to respond to Russia. He urged residents to remain calm and stay at home.

WORLD MARKETS FALL

World stock markets have plunged and oil prices surged by nearly $6 per barrel after Putin launched Russian military action in Ukraine.

Market benchmarks tumbled in Europe and Asia and U.S. futures were sharply lower. Brent crude oil jumped to over $100 per barrel Thursday on unease about possible disruption of Russian supplies.

The ruble sank 7.5% to more than $87 to the U.S. dollar. Earlier, Wall Street’s benchmark S&P 500 index fell 1.8% to an eight-month low after the Kremlin said rebels in eastern Ukraine asked for military assistance.

PUTIN’S DECLARATION OVERTAKES EMERGENCY U.N. SECURITY COUNCIL SESSION

At an emergency meeting of the U.N. Security Council called by Ukraine that opened just before Putin’s announcement, U.N. Secretary-General Antonio Guterres told Putin: “Stop your troops from attacking Ukraine. Give peace a chance. Too many people have already died.”

Guterres later pleaded with Putin, “In the name of humanity, bring your troops back to Russia.”

WHEN WILL THE WEST IMPOSE MORE SANCTIONS?

Ukraine’s forces are no match for Moscow’s military might, so Kyiv is counting on other countries to hit Russia hard — with sanctions.

Biden on Wednesday allowed sanctions to move forward against the company that built the Russia-to-Germany Nord Stream 2 gas pipeline and against the company’s CEO.

Biden waived sanctions last year when the project was almost completed, in return for an agreement from Germany to take action against Russia if it used gas as a weapon or attacked Ukraine. Germany said Tuesday it was indefinitely suspending the pipeline.

Biden said more sanctions would be announced on Thursday.

Meanwhile, the European Union planned the “strongest, the harshest package” ever, to be considered at a summit on Thursday, according to EU foreign policy chief Josep Borrell.

“A major nuclear power has attacked a neighbor country and is threatening reprisals of any other states that may come to the rescue,” Borrell said.

“This is not only the greatest violation of international law, it’s a violation of the basic principles of human co-existence. It’s costing many lives with unknown consequences ahead of us. The European Union will respond in the strongest possible terms.”

WHAT SANCTIONS WERE UNDER U.S. CONSIDERATION IF RUSSIA INVADED?

The Biden administration had made clear it was holding tough financial penalties in reserve in case of just such a Russian invasion.

The U.S. hasn’t specified just what measures it will take now, although administration officials have made clear that all-out sanctions against Russia’s major banks are among the likely options. So are export limits that would deny Russia U.S. high tech for its industries and military.

Another tough measure under consideration would effectively shut Russia out of much of the global financial system.

HOW IS UKRAINE’S ECONOMY HOLDING UP?

It was Ukraine, not Russia, where the economy was eroding the fastest under the threat of war.

One by one, embassies and international offices in Kyiv closed. Flight after flight was canceled when insurance companies balked at covering planes arriving in Ukraine. Hundreds of millions of dollars in investment dried up within weeks.

The squeezing of Ukraine’s economy is a key destabilizing tactic in what the government describes as “hybrid warfare” intended to eat away at the country from within.

The economic woes include restaurants that dare not keep more than a few days of food on hand, stalled plans for a hydrogen production plant that could help wean Europe off Russian gas and uncertain conditions for shipping in the Black Sea, where container ships must carefully edge their way around Russian military vessels.

UKRAINE SEES MORE CYBERATTACKS

The websites of Ukraine’s defense, foreign and interior ministries were unreachable or painfully slow to load Thursday morning after a punishing wave of distributed-denial-of-service attacks as Russia struck at its neighbor.

In addition to DDoS attacks on Wednesday, cybersecurity researchers said unidentified attackers had infected hundreds of computers with destructive malware, some in neighboring Latvia and Lithuania.

Officials have long expected cyberattacks to precede and accompany any Russian military incursion.

HOW HAS THE CONFRONTATION BEEN SEEN IN RUSSIA?

In the buildup to the attack, Russian state media portrayed Moscow as coming to the rescue of war-torn areas of eastern Ukraine where residents were tormented by Ukraine’s aggression.

“You paid with your blood for these eight years of torment and anticipation,” anchor Olga Skabeyeva said during a popular political talk show Tuesday morning. “Russia will now be defending Donbas.”

Channel One struck a more festive tone, with its correspondent in Donetsk asserting that local residents “say it is the best news over the past years of war.”

“Now they have confidence in the future and that the years-long war will finally come to an end,” she said.

Whether ordinary Russians were buying it is another question.