"A president who says he wants to end endless war in the Middle East is bringing us dangerously close to starting a new one," Biden said. "An administration that claims its actions have made Americans safer in the same breath urges them to leave Iraq because of increased danger."
Biden said that he was deeply familiar with Soleimani but suggested that authorising his killing was not the correct course of action.
"I have no illusions about Iran - I never have. The regime has long sponsored terrorism and threatened our interests," he said. "They've ruthlessly killed hundreds of protesters, and they should be held accountable for their actions. But there is a smart way to counter Iran - and a self-defeating way."
"Trump's approach, in my view, is demonstrably the latter," he added.
In the midst of a speech meant to remind voters of his significant foreign policy experience, Biden stumbled in referring to a vote by the "Iran parliament" to reject US troops, when he meant to reference the parliament in neighbouring Iraq.
Biden delivered his remarks in a banquet hall on a pier in Chelsea, standing in front of a large blue curtain and four American flags, an attempt to re-create the tapestry of a presidential speech in front of dozens of journalists and a handful of advisers.
He spoke in between fundraisers in New York, where he also talked extensively about the situation in Iran.
"He has to understand that he cannot take this nation to war with Iran without the informed consent of the American people," Biden said during a morning fundraiser held on the 37th floor of a Manhattan office that houses the Skadden law firm.
He said he arrived late because he was on a phone call with his national security team.
"The idea that he can take us into a conflict potentially with a country of 80 million people in the Middle East, without the consent of American people - and doing it by tweet - is just preposterous," he added.
In his speech, Biden also accused Trump of getting closer to an overreach of presidential power.
"The American people do not want, and our Constitution will not abide, a president who rules by fiat and demands obedience," Biden said.
"We need to restore the balance of powers between the branches of government," he added. "We need checks and balances that actually serve to check and balance the worst instincts of our leaders, in either of the branches. We need to use our system to bring us together as a nation, not abuse it to rip us apart."
Biden has in the past, however, been supportive of limited military action that did not require congressional approval. He sponsored legislation in 1995 that would authorise the president to use force in the absence of congressional approval to "forestall an imminent act of international terrorism directed at US citizens or nationals or to retaliate against the perpetrators of such an act."
His presidential campaign website in 2008 said, specifically about Iran, that the Founding Fathers were clear that a president needs congressional approval "to initiate war, except to repel an imminent attack on the United States or its citizens."
It also noted: "If the President takes us to war with Iran without Congressional approval, I will call for his impeachment. I do not say this lightly or to be provocative. I am dead serious."
