The number of people crossing the southern border illegally has dropped to levels not seen in decades, a sign that President Trump’s message of deterrence and his hard-line immigration policies are working to keep people out.

Border Patrol agents made just over 6,000 arrests in June, figures released this week by the Department of Homeland Security show, punctuating a steep drop since Mr. Trump took office.

Mr. Trump ran on a platform of shutting down illegal crossings of the southern border and carrying out mass expulsions of people in the United States without authorization, positions that helped sweep him into office. His campaign to execute his immigration promises has invited showdowns with the courts, tested the boundaries of constitutional rights and upended America’s longstanding role welcoming those seeking refuge or asylum.

But in his first few months in office, it also appears to have been effective.

Adam Isacson, a border expert at the Washington Office on Latin America, said the border crossings in June were the lowest since the 1960s and were likely aided by the intensified immigration push, pointing to images of arrests by Immigration and Customs Enforcement agents, the sending of immigrants to El Salvador and other countries they are not from, and the holding of migrants at the U.S. detention facility in Guantánamo Bay, Cuba.

“Smugglers’ messaging may not be able to overcome the climate of fear among the immigrant community in the United States,” he said. “People are probably getting scary messages on WhatsApp, TikTok and elsewhere about how bad for migrants the climate is in the United States.”

President Joseph R. Biden Jr.’s administration largely struggled with the southern border, with monthly arrests peaking in December 2023 at nearly 250,000. After Mr. Biden moved to limit asylum at the border, the numbers dropped under 50,000 by the closing months of his administration — but never this low.