The end of Title 42 on Thursday caps a prolonged legal battle between rights groups and US President Joe Biden’s administration, which has also faced court challenges from several Republican-led states seeking to keep the restriction in place.

Title 42 was first invoked by former US President Donald Trump in March 2020 on the pretext of stemming the spread of COVID-19. It has been used to rapidly expel people more than 2.8 million times since then, drawing condemnation from rights groups.

The Biden administration this month announced plans to send additional troops to the US-Mexico border in anticipation of the policy’s expiration, which coincides with the end of the federal COVID-19 public health emergency on May 11.

Washington has also implemented measures to discourage border crossings – including expedited screenings and deportations – that observers have said continue to raise rights concerns.

Here’s the path the policy took through the government and courts:

March 2020: The Centers for Disease Control and Prevention (CDC) under then-President Trump invokes Title 42 – a portion of US law that deals with public health, social welfare and civil rights – to allow US authorities to turn away most asylum seekers at the country’s borders.

The administration’s use of a public health measure to tighten US immigration laws is shaped by Trump’s hardline adviser, Stephen Miller, US media outlets will later report.

November 7, 2020: Biden – who on the 2020 campaign trail promised to take a more “humane” approach to immigration than Trump – is declared the winner of the US presidential elections.

November 18, 2020: A US federal court mulling a class-action lawsuit by several rights groups rules that Title 42 cannot be used to expel unaccompanied children at the border.

Lee Gelernt, a lawyer with the American Civil Liberties Union, one of the plaintiffs in the case, calls the decision “a critical step in halting the Trump administration’s unprecedented and illegal attempt to expel children under the thin guise of public health”.

January 2021: Biden takes office, but faces criticism for not immediately moving to end Title 42. The administration does, however, end the practice of expelling unaccompanied minors under the rule, per the November 2020 federal court ruling.

March 2021: US Secretary of Homeland Security Alejandro Mayorkas says the US is on pace to see the biggest increase in the number of migrants and refugees at its southwestern border in 20 years.

Mayorkas says the majority of people seeking to enter are being turned away under Title 42. Top Republicans blame Biden’s policies for the surge, while administration officials say they inherited a “broken” system.

At particular issue is the challenge of processing thousands of unaccompanied minors, who by law are required to be transferred out of Customs and Border Protection facilities to shelters within 72 hours. Photos of bleak conditions at crowded processing centres stoke further outrage.

August 2021: The Biden administration renews Title 42, saying lifting the policy would “exacerbate overcrowding at [Department of Homeland Security] facilities and create significant public health risks”.

After negotiations with the Biden administration break down, the ACLU and other rights group re-up a court challenge to the order.

September 2021: A federal judge rules that the Biden administration must stop using Title 42 expulsions against families with children, saying plaintiffs had “shown a likelihood of suffering irreparable harm”.

The Department of Justice appeals the decision, and a judge later rules the administration can continue to expel families.

April 2022: The CDC says the Title 42 order is “no longer necessary” to stop the spread of COVID-19 and will be terminated on May 23, 2022.

In line with the CDC’s decision, US Secretary of Homeland Security Mayorkas announces that the Biden administration will stop using Title 42 to expel asylum seekers at the border by that same date.

Meanwhile, the Republican governor of Texas, Greg Abbott, begins sending buses of migrants to Democratic-led cities in an effort to put pressure on the Biden administration over its border policies. Arizona will later join the campaign, as will Florida Governor Ron DeSantis.

May 2022: US District Judge Robert Summerhays issues a nationwide injunction barring the Biden administration from lifting Title 42. The ruling comes after two dozen US states sued the federal government over its plan to end the policy.

June 10, 2022: Under pressure for its push to end Title 42 and seeking to deter asylum seekers from reaching the border, the Biden administration unveils a plan it says will help nations across the Americas region address migration.

The “Los Angeles Declaration”, unveiled on the final day of the Summit of the Americas in Los Angeles, reflects what one expert dubs Biden’s “more carrots, fewer sticks” approach to immigration.

October 13, 2022: The US announces that Mexico has agreed to accept Venezuelan migrants and refugees expelled under Title 42.

The administration also says it will extend humanitarian parole – a temporary status that allows people to come to the US and work legally, but does not offer them a path to citizenship – to as many as 24,000 Venezuelans who apply from outside the US.

Tyler Mattiace, a Mexico researcher at Human Rights Watch, says the new US border policies for Venezuelan nationals are “effectively punishing those … who have been forced to flee their country on foot”.

November 16, 2022: A federal judge, responding to the lawsuit from the ACLU and other rights groups, orders the Biden administration end Title 42 within five weeks. In his decision, the judge says Title 42 is an “arbitrary and capricious” policy that violated federal regulatory law.

December 2022: Following a request filed by several Republican state attorneys general, the US Supreme Court agrees to take up the GOP-led case challenging the Biden administration’s plan to end Title 42. The top court’s decision means the policy will remain in place indefinitely.

Meanwhile, the Biden administration sees the highest number of border crossings since taking office, with US authorities encountering 251,487 people irregularly crossing the southern border during the month.

January 2023: The US announces it will end the COVID-19 public health emergency on May 11. Days later, the Biden administration tells the Supreme Court that the end of the emergency would also mean the de facto expiration of Title 42, making the ongoing court case moot.

The Biden administration also announces a plan to accept up to 30,000 people a month from Venezuela, Cuba, Nicaragua and Haiti – if they apply from outside of the country and meet certain criteria. But people from those four countries who seek to enter the US through the southern border will be sent back to Mexico – a policy again decried by rights groups.

February 2023: In preparation for the end of Title 42, Washington unveils a proposal that would bar asylum seekers from seeking protection in the US if they did not first apply and get rejected in Mexico or other countries they crossed in their journeys to the border.

Rights groups dub the proposed rule an “asylum ban” and urge the Biden administration to reconsider. Critics accuse the US president of employing a failed deterrence policy and working to extend the US border further south.

May 2023: The Biden administration announces it will open two migrant processing centres in Latin America, but will expedite screenings and deportations of those who cross the border irregularly. Sunil Varghese, policy director at the International Refugee Assistance Project (IRAP), calls the move a “Faustian bargain”.

The Department of Defense also authorises a request to send 1,500 additional US troops to the southwestern border for 90 days to help immigration authorities respond to an “anticipated increase in migration” when Title 42 expires on May 11.