Biden has said he would deal with Iran through diplomacy and re-enter the agreement, but only if Iran first returned to compliance with the deal's restrictions on its nuclear program.
NATO & TRANSATLANTIC TIES
Trump has repeatedly complained about the failure of many NATO partners to meet defense spending targets. He has also questioned the continued relevance of the organization created in 1949 at the start of the Cold War with Russia.
His attacks soured ties with several European allies, but more members of the alliance have now increased spending to meet its target of two percent of GDP.
This year, Trump vowed to cut the number of US troops in Germany, accusing Berlin of taking advantage of the United States while not meeting its NATO obligations.
Analysts say repairing the transatlantic alliance will take time, but should be one of the easier tasks awaiting a potential Biden administration.
BRINGING TROOPS HOME
Trump promised in his 2016 campaign to stay out of foreign wars and bring home US troops deployed in Afghanistan, America's longest war, which is now in its nineteenth year.
Washington has begun cutting troop numbers in Afghanistan after striking a deal with the Taliban in February that envisaged the withdrawal of all U.S. troops. This depends, however, on talks between the Taliban and the Afghan government, which have stalled.
Trump also ordered a pullout of U.S. troops from Syria. The decision was repeatedly watered down by aides and the military, but numbers have still been reduced by more than half.
PARIS CLIMATE DEAL
One of Trump's most controversial decisions was his withdrawal from the Paris Climate Agreement, something he had repeatedly vowed to do during the 2016 campaign.
Trump said the agreement imposed "draconian" financial and economic burdens on the United States and vowed to negotiate a better one.