Ukrainian President Volodymyr Zelensky pledged there would be "no compromises" in bringing an end to the war in Ukraine.
Mr Zelensky, in his first known trip out of his country since Russia invaded, paid a visit to Washington to thank US leaders and "ordinary Americans" for their support.
Just before his arrival, the US announced its largest single delivery of arms to Ukraine, the Biden administration will provide 1.85 billion dollars in military aid to Ukraine, including Patriot surface-to-air missiles.
In a speech to Congress, the Ukrainian president said the money is "not charity" but an "investment" in global security and democracy.
Mr Zelensky called the tens of billions of dollars in US military and economic assistance provided over the past year vital to Ukraine's efforts to beat back Russia and appealed for even more in the future.
"Your money is not charity," he sought to reassure both those in the room and those watching at home. "It's an investment in the global security and democracy that we handle in the most responsible way."
The speech to Congress came after President Joe Biden hosted Mr Zelensky in the Oval Office for strategy consultations, saying the US and Ukraine would maintain their "united defence" as Russia wages a "brutal assault on Ukraine's right to exist as a nation".
Congress plans to vote this week on a fresh spending package that includes about $45 billion in additional emergency assistance to Ukraine.
Mr Biden pledged to help bring about a "just peace".
The Ukrainian president rejected Mr Biden's framing of finding a "just peace", saying that a "just peace" with Russia means no compromises on his country's sovereignty and territorial integrity.
He said the war would end once Ukraine's sovereignty, freedom and territorial integrity were restored, and Russia had paid back Ukraine for all the damage inflicted by its forces.
"There can't be any 'just peace' in the war that was imposed on us," he added.
Mr Zelensky told Mr Biden that he had wanted to visit sooner and his visit now demonstrates that the "situation is under control, because of your support".
The highly sensitive trip came after 10 months of a brutal war that has seen tens of thousands of casualties on both sides and devastation for Ukrainian civilians.
Mr Zelensky travelled to Washington aboard a US Air Force jet.
This visit had been long sought by both sides, but the right conditions only came together in the last 10 days, US officials said, after high-level discussions about the security both of Mr Zelensky and of his people while he was outside of Ukraine.
The Ukrainian president spent less than 10 hours in Washington before beginning his journey back to Ukraine.
A "turning point" in the conflict
In his remarks to Congress, Mr Zelensky recalled US victories in the Battle of the Bulge, a turning point against Nazi Germany in the Second World War, and the Revolutionary War Battle of Saratoga, an American victory that helped draw France's aid for US independence.
The Ukrainian leader predicted that next year would be a "turning point" in the conflict, "when Ukrainian courage and American resolve must guarantee the future of our common freedom — the freedom of people who stand for their values".
Mr Zelensky received unanimous applause from members of Congress and presented politicians with a Ukrainian flag autographed by front-line troops in Bakhmut, in Ukraine's contested Donetsk province.
The flag was displayed behind him on the rostrum by House Speaker Nancy Pelosi and Vice President Kamala Harris.
Ms Pelosi, in turn, presented Mr Zelensky with an American flag that had flown over the Capitol that day, and Mr Zelensky pumped it up and down as he exited the chamber.
Declaring in his speech that Ukraine "will never surrender", Mr Zelensky warned that the stakes of the conflict were greater than just the fate of his nation — that democracy worldwide is being tested.
"This battle cannot be ignored, hoping that the ocean or something else will provide protection," he said, speaking in English for what he had billed as a "speech to Americans".