CNN– President Joe Biden slipped into Kyiv on Monday for the first time since Russia launched a full-scale invasion of Ukraine almost a year ago, demonstrating in dramatic personal fashion his commitment to the country and its struggle as the war enters an uncertain new phase.

The highly secretive visit – which took place as air raid sirens could be heard ringing out around Kyiv while Biden walked alongside President Volodymyr Zelensky around the gold-domed St. Michael’s Cathedral – comes at a critical moment in the 12-month conflict, with Russia preparing for an expected spring offensive and Ukraine hoping to soon retake territory.

Biden announced a half-billion dollars in new assistance, saying the package would include more military equipment, such as artillery ammunition, more javelins and Howitzers. And he said new sanctions would be imposed on Moscow later this week.

“One year later, Kyiv stands. And Ukraine stands. Democracy stands,” Biden said.

The United States and other Western nations have been rushing arms, tanks and ammunition to Ukraine in the hopes of changing the trajectory of the war. By visiting in person, Biden is offering a singular image of American support for Zelensky, who has spent the past year attempting to rally the world behind his nation and appealing for greater levels of assistance.

Biden arrived in Kyiv at 8 a.m. local time after a lengthy, covert journey from Washington and arrived to the Mariinsky Palace half-an-hour later. He departed Kyiv in the early afternoon.

“Thank you for coming,” Zelensky said, shaking Biden’s hand.

US officials have privately voiced hope the massive influx of weaponry to Ukraine – which includes new vehicles, longer-range missiles and Patriot air defense systems – can help Ukraine prevail on the battlefield and put Zelensky in a stronger position to negotiate an end to the war.

But it remains unclear what parameters Zelensky might be willing to accept in any peace negotiations, and the US has steadfastly refused to define what a settlement may look like beyond stating it will be up to Zelensky to decide.

A high-stakes visit

Biden’s visit made for a highly symbolic moment, coming a day ahead of a planned speech by Russian President Vladimir Putin marking the anniversary of the war. Speaking from Kyiv, Biden declared Putin’s “war of conquest is failing.”

“Putin thought Ukraine was weak and the West was divided,” Biden said, standing alongside Zelensky. “He thought he could outlast us. I don’t think he’s thinking that right now.”

“He’s just been plain wrong,” Biden said of Putin. “One year later, the evidence is right here in this room. We stand here together.”

During their talks inside the presidential palace, Biden laid out his rationale for visiting the Ukrainian capital as the war enters a second year.

“I thought it was critical that there not be any doubt, none whatsoever, about US support for Ukraine in the war,” Biden said.

“The Ukrainian people have stepped up in a way that few people ever have in the past,” he added.

Biden emphasized there was broad, bipartisan support in Washington for the Ukrainian cause.

“For all the disagreement we have in our Congress on some issues, there is significant agreement on support for Ukraine,” he said.

“It’s not just about freedom in Ukraine. … It’s about freedom of democracy at large,” he said

Biden was intently focused on discussing the coming months of fighting when he sat down with Zelensky, according to national security adviser Jake Sullivan.

“The President was very focused on making sure that he made the most of his time on the ground, which he knew was going to be limited,” Sullivan said. “So he was quite focused on how he was going to approach his conversation with President Zelensky and in part how the two of them were really going to look out over the course of 2023 and try to come to a common understanding of what the objectives are.”

White House officials would not go into details about the logistics of the president’s trip, saying that more details would come once they “got the green light from the security folks.”

Highly secretive trip

Biden’s trip to Kyiv was shrouded in secrecy, a reflection of the steep security concerns. Air Force One departed Joint Base Andrews under cover of darkness at 4:15 a.m. ET on Sunday, and reporters aboard the plane were not allowed to carry their devices with them.

Biden’s public schedule didn’t reflect the trip, and White House officials repeatedly said last week that a visit to Ukraine was not in the works.

Biden only decided on Friday to go ahead with the visit after huddling with top members of his national security team in the Oval Office. A few hours before he departed, the United States informed Russia of the plans to visit the Ukrainian capital for “deconfliction purposes,” according to Sullivan.

Biden was presented with a range of options for a visit to Ukraine, but decided that a trip to Kyiv made the most sense, a source familiar with the matter said. As Biden was briefed over several months on the planning for a potential visit, the source said that Biden only once expressed concern about the risk of a visit to Ukraine – but that was about the extent to which his visit could endanger others, rather than about his own safety. Other officials were obviously extremely concerned about Biden’s own safety and prepared a series of security contingency plans.

On Saturday evening, before he departed, Biden went out to dinner with his wife in Washington. He wasn’t seen in public again until arriving in Kyiv on Monday morning.

Ukraine is an active war zone where the US military has no control, making Monday’s visit different from previous presidential trips to Iraq or Afghanistan. White House officials had repeatedly ruled out a visit earlier in the year.

The Friday session in the Oval Office was the culmination of months of work by a tiny handful of Biden’s top aides to plan the trip around the anniversary of the start of the war. The planning included the chief of staff’s office, the National Security Council and the White House Military Office, with input from a small number of officials at the US Secret Service, the Pentagon and the Intelligence Community, according to deputy national security adviser Jon Finer.

Biden was regularly briefed on the plans for the trip, including threat assessments, as the visit came together.

Discussions between the White House and the “highest levels of the Ukrainian government” helped bring the trip over the finish line.

“He was satisfied the risk was manageable,” Sullivan said.

“This was a risk that Joe Biden wanted to take,” said communications director Kate Bedingfield. “It’s important to him to show up, even when it’s hard, and he directed his team to make it happen no matter how challenging the logistics.”

Biden traveled with a relatively small entourage, including Sullivan, deputy chief of staff Jen O’Malley Dillon and personal aide Annie Tomasini.

Zelensky himself traveled to Washington in December to meet Biden in the Oval Office and speak to a joint session of Congress – his first trip outside Ukraine since the war began.

The Ukrainian leader invited Biden to visit Kyiv months ago, saying he believed it was important for the US leader to see the situation up close.

Biden’s visit follows other world leaders’ trips

Biden has been itching to visit Ukraine for months, particularly after several of his counterparts in Europe all endured lengthy train journeys to meet with Zelensky in Kyiv. French President Emmanuel Macron, German Chancellor Olaf Scholz, Canadian Prime Minister Justin Trudeau and British Prime Minister Rishi Sunak, as well as former British Prime Minister Boris Johnson, have all made visits to the country to demonstrate their support.

Several of Biden’s top lieutenants, including Secretary of State Antony Blinken and Defense Secretary Lloyd Austin, have also visited the Ukrainian capital to pledge new assistance. Senior administration officials, including CIA Director Bill Burns and top White House officials, visited Kyiv last month.

Even Biden’s wife, Dr. Jill Biden, paid a surprise visit on Mother’s Day last year to a small city in the far southwestern corner of Ukraine. She met with Zelenska at a former school that was converted into temporary housing for displaced Ukrainians, including 48 children.

Now, with the war nearing its one-year mark on February 24, Biden is hoping to demonstrate to the world his commitment to Ukraine, even as it remains unclear how much longer US and western resolve can last.

Asked about the significance of being in Kyiv, Biden noted it was his eighth visit to the city. “Each time more significant,” Biden said.

He added that the purpose of his visit was to convey to Zelensky that the US is “here to stay.”

“We’re not leaving,” Biden said.

Concerns over China’s support for Russia

Biden’s visit to Ukraine also comes as US concerns grow over China’s support for Russia’s military.

American officials told CNN on Saturday the US has recently begun seeing “disturbing” trends and there are signs that Beijing wants to “creep up to the line” of providing lethal military aid to Moscow without getting caught.

The officials would not describe in detail what intelligence the US has seen suggesting a recent shift in China’s posture, but said US officials have been concerned enough that they have shared the intelligence with allies and partners at the Munich Security Conference over the last several days.

Blinken raised the issue when he met with his Chinese counterpart, Wang Yi, on Saturday on the sidelines of the conference, officials said.

Wang, who was named Chinese leader Xi Jinping’s top foreign policy adviser last month, is expected to arrive in Moscow this week, in the first visit to the country from a Chinese official in that role since Russia’s invasion of Ukraine.

According to China’s Foreign Ministry, Wang’s visit will provide an opportunity for China and Russia to continue to develop their strategic partnership and “exchange views” on “international and regional hotspot issues of shared interest” – a catch-all phrase often used to allude to topics including the war in Ukraine.

This is a breaking story and will be updated.

CNN’s Jeremy Diamond and Nectar Gan contributed to this report.