President Biden's struggles with the chaos at the border are partly of his own doing. But no president in recent history — including Donald Trump — has faced an immigration crisis of the scale that Biden has.

Why it matters: Biden was elected after promising to end harsh limits that Trump imposed on immigration. Almost immediately, the president was trapped by a humanitarian and political crisis.

• Republicans quickly seized on the issue, characterizing Biden's more lenient policies as "open borders."
• Progressives and immigration advocates who had backed Biden's election quickly proved unappeasable and critical of the administration's policy decisions, as waves of migrants poured toward the southern border.
• Current and former Biden administration officials say dysfunction and slow decision-making from the top added to what felt like an intractable situation.

The big picture: The reality at the U.S.-Mexico border over the last three years is unprecedented — back-to-back-to-back records for migrant encounters by U.S. Customs and Border Protection officials.

• Millions of people — 2.5 million crossings last year — have shown up at the border, many of them having survived journeys not just through Mexico, but from Central and South America and beyond.
• The Darién Gap — a 100-mile-long mountainous jungle separating Colombia and Panama — once "served as a real barrier for people from the rest of the world," Migration Policy Institute president Andrew Selee told Axios.
• Now, "there's a routine built up that allowed half a million people to go through last year."

Many of the migrants have come from nations that have little experience with or refuse to accept deportation flights from the U.S. — nations such as Venezuela, Cuba and Nicaragua.

• These new groups of migrants have encountered a border system stuck decades in the past. One example: Border Patrol stations designed for Mexican adults — not families from Central America.
• There's now a backlog of 3 million cases in U.S. immigration courts — more than 1 million of them involving people who have already filed for asylum.

Between the lines: When he took office in January 2021, Biden also had to deal with baggage from the pandemic and the Trump administration.

• Border numbers had already been climbing for months.
• COVID-19 restrictions limited space in shelters and detention facilities.
• Trump imposed several restrictions on asylum and curtailed the U.S. program for refugees. When Biden took office, hundreds of children remained separated from their parents because of Trump's family separation policy targeting those who illegally crossed the border.

Zoom out: Biden, now up for re-election, is caught between a rock and a hard place.

• On the left, progressive and immigration groups were morally outraged at how Trump's administration treated immigrants. They pinned their hopes on Biden, applauding his early actions to reverse Trump's policies.
• As the border crisis worsened and Biden adopted some Trump-era restrictions on immigration and asylum, Biden's relationship with those groups soured.

They criticized Biden's use of Title 42 to rapidly expel migrants at the border without a chance at asylum, as well as the president's eventual embrace of tougher border policies such as asylum limits and fast-track deportations.

• White House and Homeland Security officials initially held regular calls with advocates to seek advice and discuss immigration challenges. Those do not happen as frequently anymore, sources familiar with the situation told Axios.
• "The scale is just enormous," one former government official told Axios, describing the challenge at the border. "The advocacy world that's focused on this has not shifted with that reality."

On the right, Republicans are putting the squeeze on Biden. They have campaigned relentlessly on the "chaos at the border" — only to walk away from a bipartisan border deal in Congress that included concessions from Democrats.

• GOP support for the plan collapsed after Trump opposed the bill. The border has long been a key campaign issue for him.
• Meanwhile, House Republicans are moving toward a historic impeachment of DHS Secretary Alejandro Mayorkas for his handling of the crisis.
• The Republicans' narrative has found its mark: 70% of Americans disapprove of Biden's handling of immigration.

The bottom line: "We have a broken system in immigration," Sister Norma Pimentel, who runs migrant shelters in South Texas, told Axios.

• "It has just become such a political platform for election rather than to address the actual reality at the border."