Sources: Organization for Security and Cooperation in Europe (Russia-controlled area in eastern Ukraine); Dr. Phillip Karber, Potomac Foundation (attacks)
Note: Locations are approximate; Data as of Friday
Max Rust and Emma Brown/The Wall Street Journal

Fighting continued across several fronts Sunday, with reports of heavy losses on both sides. Ukraine’s defenders held on to Kyiv, and pushed back Russian troops in urban combat in its second-largest city, Kharkiv.

Ukrainian authorities have ordered Kyiv residents to stay indoors until Monday morning while they hunt for Russian infiltrators, who engaged in several shootouts with Ukrainian troops and civilian volunteers overnight.

Elsewhere, Russian forces were moving deeper into the country, and the White House warned that Kyiv could fall as those troops continued to advance. But they faced heavy resistance from Ukrainian troops—including recently enrolled volunteers in civilian dress—and remained behind where analysts expected them to be at this stage in the attack, raising fears that Russian forces will expand attacks on civilian targets. Ukrainian officials and witnesses said that one of the Russian airstrikes hit a children’s hospital in central Kyiv, Okhmatdyt, on Saturday night, killing one child and injuring others.

“The night was hard. What they are doing to us is revenge. It is terrorism,” Mr. Zelensky said in a speech Sunday. “They have consciously chosen to hit civilians and everything that renders life normal. Power stations, hospitals, kindergartens, housing blocks—they are all targeted daily.”

Mr. Zelensky on Sunday announced the creation of a new International Legion of the Ukrainian army, along the lines of the International Brigades that helped defend the Spanish republic in the 1930s civil war there, saying the invasion of Ukraine was the beginning of a war against all of Europe. Before the invasion, Russia had massed a fighting force totaling up to 190,000 troops along Ukraine’s borders compared with Ukraine’s 200,000-strong army, supplemented by tens of thousands of reservists.

Ukrainian civilians fleeing westward have been stuck in long lines of cars near the border with Poland. Many people have abandoned their cars and walked to the border for many hours in bitterly cold weather, carrying children and a few belongings. Poland’s border agency said Monday that more than 281,000 people had crossed over from Ukraine since the invasion began. Others are heading to the borders of Hungary and Slovakia. Filippo Grandi, the chief of the United Nations’ refugee agency, said more than 500,000 people had left Ukraine for neighboring countries.

Why did Russia invade Ukraine?

Mr. Putin says the main objective is to defend the Russian-speakers in Ukraine, especially those in the two self-declared republics of Donetsk and Luhansk, which broke away from Ukrainian control in 2014. He called it “de-Nazification,” something that political analysts see as code for an unfounded narrative Mr. Putin is trying to establish about Ukrainian politicians being puppets of the West, much as some nationalists sided with Nazi Germany during World War II.

Ukraine has denied Moscow’s claims that it is forcibly trying to take the territory back or is somehow beholden to neo-Nazis. Mr. Zelensky, the Ukrainian leader, is of Jewish origin and in an address to the Russian people before the invasion said that eight million Ukrainians died during World War II and that his own grandfather served as a Soviet officer during the conflict.

More significantly, Mr. Putin regards Ukraine and Russia as inseparable, “one people, a single whole,” and views its closer integration with the West and especially its aspiration to join NATO, the North Atlantic Treaty Organization, with deep suspicion. Ukraine was part of the Soviet Union before its collapse at the end of the Cold War in 1991, after which Russia lost much of its economic power and prestige.

But Ukrainians are increasingly speaking their own language and have been more supportive of joining the EU. Before 2014, polls showed a roughly even split in support among the population for joining the EU or a Moscow-led economic bloc. In a November survey, however, 58% favored the EU, with 21% for Russia’s group. Polling data show that even people in the south and east, where there are many ethnic Russians and Russian speakers, are now in favor of the EU.

Ukraine, though, sits on Russia’s western flank where, historically, it has been most vulnerable to attack.

Will there be peace talks?

There are indications that a negotiated end to the conflict could be possible. Ukraine sent a delegation led by its defense minister to a location on the border with Belarus where they met with a team of Russian envoys. Discussions began on Monday afternoon, just after 2 p.m. local time.

Earlier attempts to initiate talks failed after Mr. Putin excoriated Mr. Zelensky, calling him a terrorist and urging Ukraine’s military to oust him.

Is Putin planning to use nuclear weapons?

Mr. Putin on Feb. 27 raised the alert level of Russia’s nuclear forces, though not to the highest level. It appears to be the first time that Mr. Putin has publicly announced a state of combat readiness for Russia’s nuclear weapons. He previously mentioned in a documentary that aired on Russian television in 2015 that he considered placing Russia’s nuclear deterrent on alert the previous year, when Moscow annexed Crimea from Ukraine, saying Russians living on the peninsula were under threat.

Russia has more than 1,500 warheads deployed and almost 3,000 in reserve, according to an assessment published in the Bulletin of the Atomic Scientists.

Russian military analyst Pavel Felgehauer said Mr. Putin’s announcement meant that Russia’s nuclear forces were now ready to launch, shortening reaction times, but it could also be a display of brinkmanship.

White House press secretary Jen Psaki said Sunday that at no point has Russia been under threat from NATO. “This is really a pattern that we’ve seen from President Putin through the course of this conflict, which is manufacturing threats that don’t exist in order to justify further aggression,” she said. “And the global community and the American people should look at it through that prism. We’ve seen him do this time and time again.”

What are NATO and the West doing in response to Russia?

The West stepped up its response on Sunday as Ukrainian resistance held firm over the weekend and Russia intensified its assault.

The European Union on Sunday said it would finance the purchase and delivery of weapons to Ukraine in an unprecedented show of support. The EU also imposed sanctions on Belarus that are similar to those already in place against Russia. The decision to fund lethal weapons marks a sharp turn in EU policy, which until now has largely precluded deadly engagement in conflicts.

The EU and Canada on Sunday moved to bar Russian planes from entering their airspace, joining other countries in choking off Russia’s access to global aviation routes. In closing its airspace, the EU banned not only commercial airliners but any plane owned, registered or controlled by Russians, European Commission President Ursula von der Leyen said.

German Chancellor Olaf Scholz on Sunday said his country will boost military spending above 2% of GDP and create a strategic natural-gas reserve, marking a significant shift in the country’s defense and energy policies. Mr. Scholz said that he would immediately invest €100 billion, equivalent to $113 billion, in weaponry.

The United Nations Security Council on Sunday approved a rare emergency session of the broader General Assembly to consider a resolution there to push back on Russia’s invasion of Ukraine.

The early wave of sanctions included Germany’s move to halt the Nord Stream 2 natural-gas pipeline. The project is intended to enable Russia to circumvent Ukraine and other countries and pipe its gas directly into Europe at lower cost, cementing its role as a major energy supplier.

The U.S. and its partners in Europe said they would introduce sanctions that are expected to seize assets of Russia’s top leaders held in their jurisdiction, including those of Mr. Putin and his Foreign Minister Sergei Lavrov.

On Saturday, the European Union, the U.K., the U.S. and Canada introduced plans for additional sanctions that would disconnect some Russian banks from the Swift global-payment system. The exclusion will complicate Russian trade, foreign investment and remittance flows, crimping its central bank’s ability to manage the economy and limiting Russia’s access to the international financial system.

NATO allies, meanwhile, are also bolstering the alliance’s eastern flank, which borders Ukraine, deploying jet fighters and ships to the region in response. The EU has set out plans for loans and grants to Ukraine worth more than $1.3 billion. The Pentagon ordered thousands of troops to prepare for possible deployment. NATO’s Secretary-General Jens Stoltenberg called Russia’s invasion an act of war, “deliberate, coldblooded and long-planned.”

What has President Biden said?

President Biden has called Mr. Putin’s move an unprovoked, unjustified attack, and said the Russian president and his country will bear the consequences.

The U.S. president on Thursday announced plans for a wider range of punitive sanctions after introducing an initial package of measures on Feb. 22. He said the sanctions, which include cutting Russia’s two largest banks from direct access to the U.S. dollar, would crimp the Russian military’s ability to expand and restrict the country’s largest state-owned business.

“Putin’s aggression in Ukraine will end up costing Russia dearly, economically and strategically,” Mr. Biden said.

U.S. Secretary of State Antony Blinken said in a statement Saturday that Washington would provide up to $350 million in additional military aid to Ukraine, including “lethal defensive assistance” to help Kyiv resist Russian armored and airborne forces.

Is Ukraine a U.S. ally and is it part of NATO?

Since the breakup of the Soviet Union when Ukraine gained its independence, the U.S. has had diplomatic relations with the country. The U.S. is eager to see Ukraine thrive as a democracy in the face of what the State Department describes as continued Russian aggression.

Ukraine isn’t a part of the North Atlantic Treaty Organization, but it is a partner country to the military alliance, and agreements in place mean it could become a NATO member in the future. Mr. Biden has said that Ukraine’s prospective membership in NATO isn’t likely in the near term, though he said the decision ultimately belongs to the alliance.

What is the mood in Ukraine?

Following the initial shock of the invasion, many Ukranians were forced to decide whether to flee or stay and possibly fight.

By early afternoon Friday, reinforcement columns of Ukrainian armored vehicles drove into Kyiv as departing city residents, whose vehicles clogged the outbound lane, got out of their vehicles to cheer and flash victory signs. Ukrainian Mig-29 jet fighters roared low overhead, in a sign that Russia, despite its formidable advantage in aviation and two days of relentlessly bombing Ukraine’s air bases, still hadn’t achieved full control of the skies.

Ukrainian artillery and tanks were moving through the city, and thousands of volunteers lined up at recruitment centers to receive weapons. On the roads south of Kyiv, armed villagers made their own roadblocks out of tractors and sandbags.

Defense experts say the Ukrainian resistance to the Russian advances has been stronger than expected, frustrating Moscow’s attempt to seize Kyiv and topple its pro-Western government. The Russian advance toward Kyiv has been slowed by antiair and antitank weapons, raising the possibility that Russian forces might be weakened by a lack logistical support before they can achieve their objective.

—Yaroslav Trofimov, Alan Cullison, Brett Forrest, Ann M. Simmons, Daniel Nasaw, Joseph Pisani, Talal Ansari and Ginger Adams Otis contributed to this article.

This explanatory article may be periodically updated.

Write to James Hookway at This email address is being protected from spambots. You need JavaScript enabled to view it.

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