Major events in the Russian invasion of Ukraine

(Reuters) - Russia's all-out invasion of Ukraine in February, 2022, has led to Europe's bloodiest conflict since World War Two.

Kyiv's outgunned and outmanned forces withstood Moscow's initial assault and won back territory in the second half of 2022, but the failure of a Western-backed Ukrainian counteroffensive in 2023 has allowed the Russian army to reclaim the battlefield initiative this year.

On Saturday, Ukrainian President Volodymyr Zelenskiy attends a summit of at least 90 leaders and organisations in Switzerland at which he hopes to lay out a path to peace.

Russia, which has not been invited, has dismissed the initiative as a waste of time, and China, its key ally since the war began, has said it would not attend.

Following is a timeline of major events on the battlefield as momentum between the two sides has shifted back and forth:

2014: After forces loyal to pro-Russian Ukrainian President Viktor Yanukovich kill scores of demonstrators in Kyiv, he flees to Russia amid a popular uprising. Moscow responds by sending undercover troops to seize the Crimea peninsula, which Russia swiftly annexes. In eastern Ukraine's Donbas region, Russian agents and pro-Russian Ukrainian rebels launch an armed revolt and declare two independent "people's republics". Thousands are killed in fighting before the "Minsk accords" ceasefire agreements largely freeze the frontlines while leaving the status of rebel-occupied areas unresolved.

Feb. 22, 2022: After building up forces for months but denying plans for an invasion, President Vladimir Putin orders tens of thousands of Russian troops into Ukraine in a "special military operation". The invaders attack from the north, northeast and east, and make rapid initial advances, largely avoiding population centres. Western intelligence agencies predict Ukraine could fall within days, but even in the initial hours there are signs that the Russian battle plan is faltering as Ukraine repels Russian commandos attempting to seize an airbase north of Kyiv as a beachhead for a swift assault on the capital.

Feb-April, 2022: Advancing Russian forces are halted at the outskirts of Kyiv after meeting stiffer-than-expected resistance. The Russians retreat from all territory in northern Ukraine, leaving behind the bodies of civilians strewn along the streets of villages and towns - evidence of what Kyiv calls atrocities which Moscow will deny throughout the war. Moscow announces its aim is now to protect and expand the territory controlled by its proxies in the east. Ceasefire talks are held but fail to halt the fighting, with Moscow demanding recognition of its territorial gains and Kyiv vowing to recover its land.

May 20, 2022: Russian forces capture Mariupol in southeastern Ukraine after a three-month siege, during which Kyiv estimates more than 20,000 civilians were killed. The city, home to 450,000 people before the war, is largely destroyed. This completes Russia's conquest of Ukraine's southeastern coast and secures the land bridge connecting Russia to Crimea.

Sept 2022: In one of the biggest shifts of battlefield momentum of the war, Ukrainian forces make a surprise breakthrough in the northeast and recapture nearly all Russian-occupied territory in Kharkiv province. Soon after, Putin announces the annexation of four partially occupied Ukrainian provinces in the east and south, and orders a call-up of reservists, triggering an exodus of hundreds of thousands of Russian men seeking to avoid conscription.

Nov. 2022: In another major breakthrough, Ukrainian forces recapture the city of Kherson and surrounding countryside in southern Ukraine and force Russian troops to retreat across the Dnipro river that bisects the country. This will prove to be the last rapid shift in the position of the front lines before the war settles into intense, attritional trench warfare, in which attacking forces on both sides can make only slow gains against heavily fortified defenders.

Dec. 2022-May 2023: Buoyed by Ukraine's battlefield success in the second half of 2022, Western countries led by the United States pledge billions of dollars worth of weapons to arm Kyiv for a counteroffensive to win back its territory. Russia uses the winter to reinforce its defensive lines, mount a long-range missile and drone campaign against Ukrainian cities and launch a ground offensive focused on Bakhmut, a small city in the east where assaults are led by Wagner, a private army that recruits prisoners from Russian jails. Ukrainian commanders elect to defend Bakhmut rather than retreat.

March 23, 2023: The International Criminal Court in the Hague issues arrest warrants for Russian officials including Putin, wanted for the war crime of allegedly deporting Ukrainian children from occupied territory. Moscow denies the charges and denounces the court. As of June 5, 2024, Ukrainian prosecutors have opened proceedings in more than 129,000 alleged war crimes cases by occupying Russian troops.

May 20, 2023: Russian forces finally capture Bakhmut after nine months of fighting, in Europe's bloodiest land battle since World War Two. The capture of the largely depopulated and destroyed city has limited direct strategic impact, but both armies suffer crippling losses.

June 23-24, 2023: Wagner leader Yevgeny Prigozhin launches a mutiny vowing to topple Russia's military leadership which he blames for battlefield failures. His forces race towards Moscow meeting little resistance, before abruptly standing down under an agreement brokered by the leader of Belarus. Two months later, Prigozhin and top lieutenants die in a plane crash.

June-December 2023: Newly equipped with billions of dollars of Western weaponry including advanced tanks, Ukraine launches a long-awaited counteroffensive to recapture Russian-held territory. The Ukrainian forces attempt to pierce the Russian defensive line at multiple points along the southern front but fail to achieve the hoped-for breakthrough.

Spring 2024: Following the failure of Ukraine's 2023 counteroffensive, Russian forces seize the battlefield momentum, taking advantage of superior manpower and firepower. Ukraine complains that its troops are running out of weapons and ammunition, with foreign aid stalled, including a $61 billion package held up by Republicans in the U.S. Congress. After months of delay, the U.S. House of Representatives finally approves the aid on April 21. In May, Russian forces open a new front with an assault on Ukraine's northeastern Kharkiv province, which Kyiv's commanders say risks diverting resources from defending against the main Russian effort in the east.

May 2024: A new mobilisation law comes into force that will make it easier for Ukraine to call up more men to fight. The number of new recruits the legislation will provide is unclear, but it could involve hundreds of thousands of people.

(Compiled and written by Peter Graff; Editing by Mike Collett-White and Gareth Jones)