Joe Biden embraces Paul Whelan, who was released from detention in Russia. / Reuters/Nathan Howard

WASHINGTON (Reuters) - The largest prisoner exchange between Russia and the West in decades could help U.S. President Joe Biden burnish his foreign policy legacy in his waning months in office and boost Vice President Kamala Harris in her bid for the White House.

But, from the perspective of Washington, it came at a significant cost: the freeing of Russians convicted of serious crimes in exchange for Americans the U.S. deems unjustly detained, a trade-off some experts say could encourage hostage-taking by U.S. foes. That concern quickly exposed Biden to attacks from Republican opponents.

Though Biden's record on the world stage is likely to be heavily defined by the conflicts in Ukraine and the Middle East, the complicated, multi-country prisoner swap with Moscow on Aug. 1 provided a much-needed foreign policy accomplishment amid heightened global tensions.

It was all the more difficult to reach a landmark deal against the backdrop of the war in Ukraine, in which the U.S. is arming Kyiv against a Russian invasion, sending relations between Moscow and Washington to the lowest point since the Cold War.

The prisoner swap goes a long way, however, toward addressing what Biden's aides have long identified as a key priority on his foreign policy agenda, especially after he ended his reelection bid and endorsed Harris for the Democratic nomination.

"It is an achievement. Smart diplomacy produced it," said Dennis Ross, a former Middle East adviser in Republican and Democratic administrations. "Getting back Americans seized and held unjustly is part of the responsibility any government has."

Biden on Thursday hailed the prisoner swap, including the release of Wall Street Journal reporter Evan Gershkovich and ex-Marine Paul Whelan, as a "feat of diplomacy," and by extension it could give Harris a positive note to sound on the campaign trail against Republican nominee Donald Trump.

But experts say Biden will have to fend off Republican criticism of the deal and, in a broader sense, will still have plenty of work to do to polish his foreign policy record for the history books.

BIDEN FOREIGN POLICY: A MIXED BAG

On the plus side, Biden is widely credited with repairing alliances with NATO and key Asian partners such as Japan and South Korea following Trump's 2017-2021 presidency, during which he cast doubt on the value of those ties.

As a result, some of America's friends are now nervous about the prospects of Trump reclaiming the White House in the November election.

Biden also faces a range of international crises that erupted on his watch.

Ukraine's fight against Russia remains unresolved. Israel's war against Hamas militants in Gaza rages on amid growing fears of a broader regional war.

China continues to pose a threat to self-ruled Taiwan and a challenge to U.S. interests in the Indo-Pacific. After an election on July 28 that the Biden administration had pushed for, Venezuelan President Nicolas Maduro has claimed victory despite exit polls that showed an overwhelming opposition win.

And still overshadowing Biden's record from his first year in office is the chaotic U.S. withdrawal from Afghanistan and the Taliban's re-establishment of control.

Republicans have seized on a campaign message of a globe engulfed in chaos on Biden's watch – what some of them have called a "world on fire" - and are trying to paint Harris with the same brush.

Biden's allies contend that every U.S. leader has had to confront his own set of challenges on the world stage, most not of his making, and that the president has handled them skillfully.

GERMANY'S CRITICAL ROLE

Two weeks ago, the Biden administration got formal word that Russia was ready to accept the deal, aides said.

It was happening even as Biden's own political career was falling apart, with the president working the phones on the issue just hours before he released a letter to the public that he would no longer seek reelection.

The prisoner swap finally came together after more than a year of painstaking negotiations. It also involves Germany and other European allies, with 24 prisoners changing hands, including the return of four to the U.S. and eight being sent to Russia.

Crucial to the deal was Germany agreeing to Russian President Vladimir Putin's main demand for the release of Vadim Krasikov, who was convicted for the murder of a former Chechen militant.

Jeff Rathke, a former U.S. diplomat and president of the American-German Institute at the Johns Hopkins University, said the exchange reflected much-improved relations between Germany and the United States under Biden compared to Trump.

The German government consented, he said, "because the United States and Germany had a level of confidence, trust and mutual interest that allowed something difficult in the German justice system to occur and could cause criticism at home."

However, there is no sign that Putin is looking to repair ties with Washington, and Biden administration officials said the deal appeared to be a one-time exchange.

Republicans wasted little time in accusing Biden of paying too high a price, as they did in response to the 2022 swap of U.S. basketball star Brittney Griner for convicted Russian arms Viktor Bout.

"I remain concerned that continuing to trade innocent Americans for actual Russian criminals held in the U.S. and elsewhere sends a dangerous message to Putin that only encourages further hostage taking by his regime," Michael McCaul, chair of the U.S. House Foreign Affairs Committee, said in a statement.

Brett Bruen, a former foreign policy adviser in the Obama administration, said that while the exchange was a significant short-term achievement, several similar deals reached during Biden's term had "upended decades of practice that dictates that you don't negotiate with tyrants for the release or Americans."

Griner's return was followed last September by the swap of five detainees each between the U.S. and Iran. That included the transfer by the U.S. of $6 billion of Tehran's frozen funds to a Qatari bank account earmarked for humanitarian purposes, which Republicans said was essentially paying "ransom."

And in December, Venezuela released 10 Americans in exchange for the U.S. release of a jailed Maduro ally.

Administration officials have defended such deals as painful but unavoidable when all other means have failed.

"While it is or would be great to have these individuals released, it underscores how hostage-taking has become a prominent and frequent - if not growing - element of Russian strategy toward the U.S. and the West," said Ian Brzezinski, a former U.S. defense official.

WHY NOW?

It was unclear why Putin decided to work out a big prisoner swap with Biden instead of holding off for the possibility that Trump – who has shown himself more amenable to Moscow's interests - might be back in the White House in January.

But Putin may have calculated that it was better to do business with Biden instead of waiting for the next administration and possibly having to start from scratch.

U.S. officials believe it was a one-off deal because of Germany's willingness to participate and there was no guarantee such an offer would still be on the table under the next president.

Trump had repeatedly insisted that, if elected, he could easily get Gershkovich released, writing in May that Putin "will do that for me, but not for anyone else, and WE WILL BE PAYING NOTHING!"

Dozens of U.S. nationals are still what Washington terms "wrongfully detained" or held hostage by foreign governments, including some in Russia and others in China and Iran, or with non-state actors such as the Palestinian militant group Hamas.

While Biden promised to continue seeking their release until he leaves office on Jan. 20, it is likely the task will be inherited by the next president.