Originally Published: 23 MAR 22 00:02 ET

Updated: 23 MAR 22 12:52 ET

By Kevin Liptak, CNN

(CNN) -- President Joe Biden departed Wednesday on one of the highest-stakes presidential trips in recent memory, a moment for the US President to assume leadership of a newly united West.

His visits to Brussels and Poland could still underscore the alliance's limits in ending the bloodshed in Ukraine, with Western leaders struggling to find ways to halt Russian President Vladimir Putin's war. So far, punishing Western sanctions haven't stopped Putin, and it's unclear whether the new steps expected this week -- including sanctions on hundreds of members of Russia's lower legislative body and changes to NATO's force posture along its eastern edge -- will be different.

Yet as he departed the White House early Wednesday, Biden appeared intent on using his last-minute wartime visit to Europe to send a message. Asked what he'd say to his partners, Biden said he'd wait to deliver it face to face.

"All I have to say, I'm going to say it when I get there," he said before boarding a seven-hour flight to Belgium.

Emergency summits of NATO, the European Union and the G7 will focus on displays of cooperation in punishing Russia and providing support to Ukraine as it comes under fire. A stop afterward in Poland is meant to highlight the massive refugee crisis that's followed Russia's invasion as well as to reassure allies on NATO's eastern edge.

For Biden, the last-minute talks are a venue to demonstrate the foreign policy credentials he promised as a candidate, when he vowed to restore American leadership and repair broken alliances. The war in Ukraine is widely viewed inside the White House as one of the defining challenges for Biden and his presidential legacy. An emotional challenge from Ukrainian President Volodymyr Zelensky, issued last week during his virtual address to Congress, now hangs over Biden's entire trip: "Being the leader of the world means being the leader of peace."

The White House has ruled out Biden visiting Ukraine itself after leaders there said it would make for a powerful symbol amid the war. Instead, he will issue his most direct efforts at Western unity during the war so far, rallying his counterparts at a critical juncture for the crisis.

Military officials say Russia's weeks-old invasion has reached a stalemate, leading to fears over how Putin will proceed. Among the topics for leaders to discuss this week is how to respond should the Russian President decide to use nuclear or chemical weapons or launch a massive cyberattack.

Asked as he was leaving the White House about the risk of chemical warfare, Biden said he felt it was "a real threat."

Earlier in the week, Biden said Putin's stumbles in Ukraine would cause him to resort to more troubling options, with cyber attacks being his primary concern.

"Now Putin's back is against the wall. He wasn't anticipating the extent or the strength of our unity. And the more his back is against the wall, the greater the severity of the tactics he may employ," Biden warned during remarks to business leaders before departing Washington.

Biden looks to spur allies to further action against Russia

Biden hopes to emerge from an intensive day of meetings Thursday with some new actions to announce alongside his partners: fresh sanctions on Russia, steps to cut off its oil and gas profits, or new announcements of military or financial assistance to Ukraine.

He plans to put sanctions this week on hundreds of Russians serving in the country's lower legislative body, an official familiar with the announcement said, setting off what are expected to be many new steps to punish Russia for its war in Ukraine. The US had already sanctioned some members of the body, but this week's announcement will expand the list.

Ahead of Biden's trip to Brussels, the Pentagon provided the White House with a series of options for potential additional US troops in eastern Europe, according to a US official. Biden could announce changes to the force posture following his meetings Thursday, though any announcement would depend on conversations with allies and is not finalized.

US Ambassador to NATO Julianne Smith said Wednesday that NATO leaders would approve at the summit the deployment of four additional battle groups in Hungary, Romania, Bulgaria and Slovakia. Speaking at an Atlantic Council event, Smith said the battle groups were "not just national forces" and said additional details would be shared on Thursday. She added that NATO is in discussions about its medium- and long-term force presence in its eastern flank.

"We're going to have to make a series of ongoing assessment about our force posture that will take us weeks and months into the future," she said, noting that "it's still an open question" of how the alliance takes the NATO-Russia Founding Act -- which NATO allies believe Russia is "in clear violation of" -- and moves to its longer-term posture.

National security adviser Jake Sullivan said a day ahead of Biden's departure that the President would make a slew of new announcements, including "a joint action on enhancing European energy security and reducing Europe's dependence on Russian gas at long last," as well as "longer-term adjustments to NATO force posture on the eastern flank."

Biden will also make further commitments on human rights "to respond to the growing flow of refugees" from Ukraine.

And Sullivan said a new package of sanctions would be unveiled Thursday "in conjunction with our allies."

"One of the key elements of that announcement will focus not just on adding new sanctions, but on ensuring that there is a joint effort to crack down on evasion, on sanctions busting, on any attempt by any country to help Russia basically undermine, weaken or get around the sanctions," he said, calling that "an important part of this next phase."

But Western leaders are quickly confronting the limits of what they can do to counter Putin.

Biden and his Western allies have largely ruled out using their own forces in Ukraine, wary of entering the conflict directly. They have rebuffed calls from Zelensky to establish a no-fly-zone or provide fighter jets.

Even a Polish proposal to establish an international peacekeeping force in Ukraine, which Warsaw is expected to raise during Thursday's meeting, has been met with skepticism from US officials, who say Biden is opposed to any scenario that would pit American troops directly against Russians.

Still, there remain some items for leaders to adopt. White House and European officials have spent the past week in intensive discussions to develop a menu of options, and Biden discussed the summit with his counterparts from the United Kingdom, France, Germany and Italy on Monday.

"They can do a couple of things. One, they can turn on the spigot. The weapons are flowing. But the spigot can be bigger. It can be wider open. There can be more going in there," said William Taylor, a former US ambassador to Ukraine.

"All the kinds of things we talk about in terms of anti-air, air defense, anti-armor, the Stingers, all those can go in. That aperture needs to be wider. That's number one. Number two, tighten the noose. There are still sanctions that are not in place yet."

That includes expanding the list of Russian banks cut off from the SWIFT international banking system, imposing sanctions on more Russian oligarchs or politicians, or taking further steps to cut off Russia's energy profits.

Biden announced earlier this month that the US would ban imports of Russian oil, natural gas and coal, though those products represented only a small portion of American energy use. Far more impactful would be a European move in the same direction.

The White House has been working with other major oil- and gas-producing countries to divert supplies to Europe, and the European Union announced steps in conjunction with Biden to reduce its dependence on Russian energy. The topic is expected to arise in conversation at this week's summits in Brussels.

Biden unlikely to visit Zelensky as diplomacy ramps up

More perplexing for the leaders gathering in Brussels this week could be the halting attempts at diplomacy between Russian and Ukrainian negotiators, which are viewed skeptically by many in Washington and Europe.

Zelensky has recently indicated he is willing to consider some concessions to Russia to help bring an end to the violence, including a neutrality policy -- albeit one underpinned by robust security guarantees -- raising more questions about the state of talks and specific elements of any peace deal that may be under consideration.

NATO leaders have been discussing whether and how Zelensky should participate in this week's summit. Ukraine is not a member of NATO, which has allowed the US and other members to set limits on how far they are willing to go in defending it. Yet Zelensky has proved a powerful international voice and leaders are mindful of his response to their decisions.

Since last month, Zelensky has been asking Biden to visit Kyiv, suggesting it would make for a dramatic show of support. But the option was never explored at the White House.

"Any president of the United States traveling into a war zone requires not only security considerations but also an enormous amount of resources on the ground, which is always a factor for us as we make considerations," press secretary Jen Psaki said this week. "It was a decision made about what would be most effective on the trip."

Ukrainian officials said Biden could show his support in other ways.

"We wish him to come to Ukraine, but unfortunately that's not the case this time," Igor Zhovkva, deputy head of Zelensky's office and his chief diplomatic adviser, told CNN's Christiane Amanpour this week.

"We are thankful to the US President for the level of resistance he's now rendering. Really -- this is a clear sign of support, not only politically and by words, but in terms of military equipment, in terms of weapons and ammunition, in terms of sanctions."

Still, Zhovkva said leaders at this week's summit could make even more efforts at getting Ukraine what it needs to sustain the battle against Russia, including anti-missile and anti-aircraft systems and fighter jets. And he said Biden's example would matter most as the world looks to him for leadership.

"This is where the political position, the political will of the US matters," Zhovkva said. "We do know which countries do possess this weapon. Sometimes they look at the US, they look at the position, they start to play pingpong. Again, the will and courage of US President is what matters now."