It was one in a series of stark declarations that framed a sweeping plan to crack on illegal immigration following his more measured tone earlier in the day, when the Republican nominee huddled with Mexican President Enrique Pena Nieto.
"We will break the cycle of amnesty and illegal immigration," Trump declared to a rowdy crowd in Phoenix.
Immigration is Trump's signature issue as he battles Democrat Hillary Clinton ahead of the November 8 election, and he laid out a tough, 10-point plan to end illegal border crossings and abusive behavior by undocumented people in the country.
"Our message to the world will be this: you cannot obtain legal status or become a citizen of the United States by illegally entering our country," he said.
"You can't just smuggle in, hunker down and wait to be legalized," he added. "Those days are over."
The fiery speech confirmed Trump's hard line on immigration shortly after he insisted to Pena Nieto that a giant wall would be built on the US-Mexico border if he is elected.
The billionaire candidate's plan includes deporting immigrants with criminal records, cancelling President Barack Obama's executive orders protecting millions of undocumented migrants, and blocking federal funding to so-called "sanctuary cities" that bar discrimination against the undocumented.
Clinton has expressed support for a pathway to citizenship for most of America's undocumented.
Trump is said to have been mulling whether to soften his hardline policies, particularly his early call to deport some 11 million undocumented migrants living in the shadows.
He has vacillated between reaching out to minorities and returning to the anti-immigration rhetoric admired by his most ardent supporters, mainly white working-class males.
His tense international trip on Wednesday saw Trump navigating a political tightrope just 69 days before the US election.
While he insisted -- as he regularly does on the campaign trail -- that Mexico will pay for the wall, he said that he and Pena Nieto did not discuss who would fund the construction.
But Pena Nieto contradicted Trump, tweeting that he told the Republican nominee in their meeting that Mexico would not pay for such a wall.
"At the start of the conversation with Donald Trump I made it clear that Mexico will not pay for the wall," Pena Nieto wrote after Trump departed Mexico City for Phoenix.
Later in Phoenix, Trump painted many illegal immigrants as murderers, which served as red meat to his mainly white supporter base.
He doubled down on the deeply complex issue by laying out a sweeping plan to dramatically slash illegal immigration, and issued direct threats to those in the country without documentation.
Political analyst Bakari Sellers, who supports Clinton, branded the speech "nationalism on steroids."
Trump demanded an end to "catch-and-release" programs along the country's southern border, while calling for stricter enforcement of existing immigration law and "zero tolerance for criminal aliens."
He also promised to triple the number of immigration officers responsible for deportation. "I am going to create a new special deportation task force focused on identifying and quickly removing the dangerous criminal illegal immigrants in America who have evaded justice," he said.
But he remained unclear about exactly what would become of the 11 million people currently living in the shadows.
"For those here illegally today who are seeking legal status, they will have one route and one route only: to return home and apply for re-entry like everybody else under the rules of the new legal immigration system that I have outlined above," Trump said.
It was a stark contrast to his relatively measured tone during his surprise visit to Mexico, where he sought to portray himself as a capable statesman on the international stage after a year of lobbing insults across the border.
The Trump campaign described the visit as "the first part of the discussion and a relationship builder," not a negotiation.
Clinton pounced, with her team arguing that Trump "lied" by saying the pair did not discuss who would pay for the wall.
"Trump just failed his first foreign test," Clinton tweeted.
"Diplomacy isn't as easy as it looks."
Trump launched his campaign last year by declaring that Mexico was sending "rapists" and other criminals across the border. Afterward, Pena Nieto likened Trump's isolationist positions to those of Nazi leader Adolf Hitler and fascist Benito Mussolini.
But the two appeared cordial on Wednesday. Trump said he believed "Mexico will work with us" to reduce illegal immigration, drug trafficking, and cross-border weapons smuggling.
Still, Pena Nieto did not hide the fact that many in his country have felt the sting of Trump's harsh rhetoric.
"Mexican people felt hurt by the comments made," he said alongside Trump. "But I'm sure that the genuine interest is to build a relationship that will give both of our societies better welfare."
Others were less interested in dialogue.
"Trump not welcome in Mexico, not by me nor the 130 million Mexicans," the country's former president Vicente Fox tweeted.