MOSCOW — Talks between U.S. and Russian officials Monday in Geneva open a round of diplomatic meetings seeking to defuse tensions over Moscow's military buildup on the border with Ukraine. The challenges to finding common ground are clear.

Russian President Vladimir Putin has warned that he will find his own "military technical" solution if NATO does not stop its "aggression." The Biden administration and its allies have stood by Ukraine and its aspirations of deepening its bonds with the West - although making clear that Ukraine will not be admitted to NATO membership any time soon.

Putin has shown an ability to rattle NATO leaders and keep Europe off balance, demanding sweeping security guarantees that include NATO's ruling out any future expansion in Ukraine or other countries along Russia's borders. NATO leaders say Moscow cannot dictate the alliance's moves or undercut its open-door policy on membership.

Russia's massing of troops and equipment near Ukraine twice over the past year has raised U.S. intelligence fears of an invasion, confronting President Biden with his most pressing security crisis.

How did we get here? As U.S.-Russian relations plummeted this past year, Moscow kept tightening its pressure and ramping up its warnings.

Here is a chronology:

1. March 17: 'Killer' comment. In a TV interview, Biden agreed that Putin was "a killer," comments that outraged Russia and prompted it to temporarily recall its ambassador, Anatoly Antonov. In the months that followed, Putin, facing opposition protests and criticism at home, steadily jacked up anti-American and anti-Ukrainian sentiment with warnings that Russia was threatened by outside enemies.

2. March 30: Military moves. In March, Russia launched a massive military buildup near its border with Ukraine, prompting Ukraine's then-military chief, Ruslan Khomchak, to warn that the Russian action threatened Ukraine. In April, Russia withdrew some forces but left much of its equipment in place. By October, Russia had resumed its military buildup near Ukraine.

3. April 6: Missile tests. Russia sent a message of military strength, launching a Kalibr cruise missile in the Sea of Japan on April 6. Russia also conducted tests of its hypersonic Tsirkon missile throughout the year. Putin has said Russia's hypersonic missiles are world-leading and impossible to deter.

4. April 15: Sanctions and expulsions. Biden announced the expulsion of 10 Russian diplomats and put sanctions on 32 individuals and companies accused of interference in the 2020 U.S. presidential election and other actions including spreading disinformation - adding to earlier sanctions over the near-fatal poisoning of Russian opposition leader Alexei Navalny in August 2020. (Navalny is imprisoned in Russia, having returned home after treatment in Germany.) Russia responded with its own expulsion of American diplomatic personnel. U.S. Ambassador John Sullivan left Russia temporarily.

5. June 16: Summit. Biden and Putin held a summit in Geneva, agreeing on little except to pursue talks on arms control and cybersecurity. The Kremlin called the meeting "rather positive." But on June 23, the British warship HMS Defender sailed close to Crimea, which Russia annexed from Ukraine in 2014, a signal to Moscow that NATO does not recognize the waters as Russian. Russia said it fired warning shots, and Putin said later that he could have sunk the ship if he wanted.

6. June 30: NATO exercises. On the eve of NATO's Sea Breeze naval exercise with Ukraine in the Black Sea, Putin said Western military support for Ukraine creates "significant security problems" for Russia.

7. July 12: Putin's manifesto. Putin prepared the ground for possible renewed aggression against Ukraine in a long essay, "On the Historical Unity of Russians and Ukrainians," claiming that Russia and Ukraine were "one people - a single whole." He argued that Ukrainian sovereignty was "possible only in partnership with Russia." And he accused the West of using Ukraine as an aggressive "anti-Russia project," akin to "the use of weapons of mass destruction against us."

8. Aug. 31: Afghanistan chaos. The fumbled U.S. withdrawal from Afghanistan was taken by the Kremlin as a telling sign of American decline, signaling that Washington was unlikely soon to commit forces in distant places.

9. Sept. 10: Zapad drills. Russia's held its massive Zapad 2021 military exercises, demonstrating a formidable fighting force after years of modernization. The exercises came as Russia was increasingly irritated by NATO flights and naval patrols on its western border.

10. Oct. 6: Espionage allegations. NATO announced the expulsion of eight diplomats in Russia's NATO mission for alleged spying, and Russia swiftly suspended its NATO mission and closed NATO's Moscow office. In late October, Ukraine for the first time used a Turkish Bayraktar drone in its war against Russian-backed separatists in the country's east. By the end of the month, Russia had resumed its military buildup near Ukraine, sparking new fears of a massive invasion.

11. Nov. 18: Russia's 'red lines'. Putin accused the West of ignoring "our warnings about red lines," referring to Ukraine's aspirations to join NATO. He said Russia's hard-line policies were working. "Our recent warnings have been heard and the effect is noticeable. Tensions have risen," he said. He added that Russia should maintain the tensions "as long as possible."

12. Nov. 30: NATO defenses. Putin complained about NATO's missile-defense system in Romania and the alliance's plans for a similar network in Poland. Putin warned that Moscow would never accept the deployment of missile systems in Ukraine.

13. Dec. 7: Biden's warnings. In a video meeting, Biden warned Putin of tough economic sanctions if Putin invaded Ukraine. Russian presidential aide Yuri Ushakov said Putin called for binding security guarantees from Biden, including a demand that NATO would not expand to the east. Russia released its list of demands Dec. 17.

14. Dec. 23: Putin's demands. Putin intensified his combative rhetoric. He said that the West had brought missiles "to our doorstep" and must offer security guarantees to Moscow "right now."

15. Dec. 30: Second call. Biden and Putin held their second call in a month, at Putin's request. Biden repeated his warning of unprecedented Western sanctions if Russia attacked Ukraine. Putin warned that such action by the West would cause a complete rupture in relations. Ushakov, the Russian presidential aide, said Russia was not interested in compromise or drawn-out talks and would insist on its security demands.