Coming face to face with European allies, almost exactly one month since Russia's invasion began, US President Joe Biden was determined to send a message of unity.
Meetings of NATO, the European Council and the G7 which would normally be planned well in advance were hastily arranged in Brussels to discuss one of Europe's most serious security crises since World War II.
"[Vladimir] Putin was banking on NATO being split," Mr Biden told reporters at a press conference in between meetings.
"Putin is getting exactly the opposite of what he intended to have as a consequence of going into Ukraine."
NATO has agreed to again ramp up the military presence along its eastern flank, sending in four new battle groups to Slovakia, Romania, Bulgaria and Hungary.
But with no clear picture of how the war will unfold from here, and amid warnings of the potential use of chemical or even nuclear weapons, experts warn the world is facing a "pivotal and dangerous moment".
"Maybe it's not a direct war, but it certainly is a proxy war, in terms of their efforts to make Putin pay a price.
"And as a result of that, it is critical that they continue to have these meetings, urgent meetings, to make sure everybody's on the same page.
"Because Russia, and Putin, will look for any break in that unity, any weakness in that unity."
US preparing for possible chemical attacks as Biden warns 'we would respond'
The US President has repeatedly warned Russia could be planning the use of chemical weapons in Ukraine, describing it as a "real threat" as he departed Washington for Europe.
The White House has reportedly begun assembling a group of experts, known internally as a Tiger Team, to prepare for the possibility of chemical, biological or nuclear attacks.
But what is still not known publicly is what the US would do in the event that Russia crosses that line.
"We would respond if he uses it," Mr Biden said when asked whether a chemical attack would trigger military action from the US and NATO.
"The nature of the response would depend on the nature of the use."
Mr Panetta said a range of options were likely being considered.
"But the bottom line is, if Russia resorts to these kinds of weapons, they're the ones that have escalated that conflict," he said.
"And both the United States and our allies have a responsibility to respond."
More sanctions and aid but not the support Ukraine is looking for
Mr Biden went into the Brussels meetings with a series of announcements, pledging $US1 billion in humanitarian aid along with a promise to re-settle up to 100,000 Ukrainians.
The US has also imposed sanctions against another 400 Russians, including members of the Duma, while the European Union and the G7 have vowed to crack down on sanctions being evaded.
But in a virtual address to the NATO summit, Ukrainian President Volodymyr Zelenskyy appeared frustrated as he again appealed to the alliance for more military support.
"You have at least 20,000 tanks. Ukraine asked for 1 per cent of all your tanks.
"The worst thing during the war is not having clear answers to requests for help."
Tentative plans to send Soviet-era fighter jets into Ukraine stalled after a disagreement between the US and Poland on how to make it happen.
Ukrainian pilots are only trained to fly the older style planes, but Poland’s offer to send its fleet via an American airbase in Germany took the US by surprise.
The US quickly rejected the proposal, fearing it could be viewed by Russia as a significant provocation at a time when Mr Biden keeps warning of the need to avoid "World War III".
And while the US is sending billions of dollars' worth of military equipment to support the Ukrainian army, requests for a no-fly zone have been repeatedly refused out of fear it would bring the West into direct conflict with Russia and risk a broader war.
Mr Zelenskyy did not even raise it in his address to NATO members.
Four weeks into the war, the response by Western nations has taken even Russia by surprise and Mr Biden's challenge now is to maintain that sense of coordination.
Just how far Russia is prepared to go though, and how many Ukrainians will suffer as a result, is still frighteningly unclear.
