Washington and Tehran share the goal of stopping Syrian-based militants in Iraq. But experts say it's Iran, not the United States, that will most influence how the new government in Baghdad approaches internal reforms and the war in Syria.
U.S. airstrikes against Islamic State militants have helped Iraqi troops recapture the country's biggest dam. But driving those fighters back across the Syrian border will require a bigger push by the new government in Baghdad, says U.S. Institute of Peace analyst Steve Heydemann.
"The U.S. has made very clear that it views this problem, the problem of ISIS, as centrally a problem of the central government in Iraq and not as a problem that the U.S. can solve," he said.
Meeting with Prime Minister-designate Haider al-Abadi, German Foreign Minister Frank-Walter Steinmeier says he must bring Iraqi Sunni, Shia and Kurds together against the militants.
"There is great hope connected with him that he will be the one who can unite the various regions and religions and can represent them all in his government," he said. "That is necessary because it is the only way to strip the terrorist group ISIS of the support of the many dissatisfied."
Sunni leaders considering whether to support the new government say there must be changes to Shia-dominant security forces before taking on Sunni militants.
Dulaimi Tribal Head Ali Hatem Suleiman says al-Abadi cannot begin by asking Sunni to fight the Islamic State.
"The Islamic State is not a big problem for us or the main issue that we suffer from," he said. "He has to purge first the security and intelligence apparatus."
And that involves Iran, which helped the previous government create that situation, and will continue to influence much of what happens in Iraq, says Steve Heydemann.
"It doesn't much matter to them which Shia is the prime minister in Iraq. Iran will continue to wield influence in Iraq,' he said. "And on questions of broader regional policy, including Iraq's rather quiet, but nonetheless consistent, support of the Assad regime will continue."
Iran is not only backing Syria's government forces, it's using Syria to arm Hezbollah militants.
American University professor Guy Ziv says greater cooperation between Washington and Tehran may change that.
"There has been shipment after shipment -- many of them intercepted by Israel -- from Iran via Syria to Hezbollah in Lebanon," he said. "And I think that one of the hopeful situations with the improved U.S./Iranian relationship is to perhaps break that axis."
But having a common enemy in the Islamic State will not bridge fundamental differences between Washington and Tehran, says former U.S. Ambassador Adam Ereli.
"If you look at any issue -- Hamas, Syria, Iraq, the nuclear file, human rights inside Iran -- there is a consistency of behavior and a consistency of policy that doesn't change," he said. "And it is a policy that is innately hostile to our interests and the interest of our allies."
Haider al-Abadi will need both Iran and the United States to effectively fight the Islamic State and convince Iraqi Sunni and Kurds to back his new government in Baghdad.