The news that the mainstream Palestinian group Fatah has agreed to form a unity government with the militantly Islamist Hamas may move some to dismay. Although there are ample reasons for that reaction, this development may also present an opportunity.

Over the past year, the world has changed not just around Israel, but also around the Gaza Strip and West Bank, the two Palestinian territories ruled, respectively, by Hamas and Fatah. That is especially true for Hamas. The Arab Spring has driven one of the movement’s two most important sponsors -- Syria -- into turmoil. The crackdown by Bashar al-Assad’s regime on the country’s Sunni majority has unnerved many inside Hamas, which is also Sunni, and its chairman-in-exile, Khaled Mashaal, has left his headquarters in Damascus. He also recently announced he would not serve another term.

Meanwhile, Hamas’s main financial backer -- Iran -- faces increasing economic constraints as a result of international sanctions. That relationship is strained, too, not least because Shiite Iran is backing the Syrian regime. Just as important, other Islamists that share Hamas’s roots in the Muslim Brotherhood have come to power in Egypt and Tunisia. More may follow. As fraught with risks as those changes are, they also have the potential of creating more moderate patrons and role models for Palestinian Islamists.

Hamas remains a radical Islamist organization defined by its implacable opposition to Israel. There is no reason to think Hamas is about to sign up to all three principles that the Middle East Quartet -- the U.S., the European Union, the United Nations and Russia -- have set for the group to receive international acceptance: namely, commitments to nonviolence, recognition of Israel and assent to previous agreements.

There is evidence, however, that the movement is re-evaluating its friends and options and that at least some of the leaders in this fractious organization are experimenting with a more pragmatic tone. Hamas’s agreement to share power with secular rival Fatah is itself something of a concession.

All of this leaves policy makers in the U.S. and Israel with two broad options: They can seize on these developments as a moment of weakness for Hamas and seek to reinforce its isolation, thereby preserving the status quo; or they can work with governments that have open communications with Hamas, such as Turkey, Qatar and Jordan, as well as the Muslim Brotherhood in Egypt, to encourage Hamas onto a more moderate path. At this particular moment, the latter seems a policy worth exploring.

Isolation has succeeded in keeping Hamas militarily weak, but on other counts the policy has failed. Notably, it ensured that Hamas remained in the willing arms of Iran, and an economic blockade failed to stir revolt inside Gaza. Hamas is unlikely to fold up and disappear any time soon.

There are clear limits to what countries can or should do to work with Hamas until it signs up to the three principles. But Washington, in particular, should look for ways to avoid torpedoing the newly merged Palestinian Authority by cutting off its funds, and instead wait to see whether the alliance has a moderating effect.

The U.S. administration wisely declined to criticize the preliminary deal announced Monday by both factions, while repeating the Quartet principles for any future Palestinian government to continue receiving U.S. support, including about $500 million in annual aid. The EU, already on Monday, said it would continue its $600 million of aid.

The details of the Fatah-Hamas agreement have yet to be worked out. If, as billed, it produces a truly technocratic government without parties, then the U.S. and other donors should use this creative ambiguity to allow funding to continue. The threat of losing money could then be used to influence Hamas’s behavior.

This would represent a significant change in policy. The U.S. lists Hamas as a terrorist organization and has worked hard to isolate it. But at a time when so much in the Middle East is changing and Hamas is under pressure to change with it, this is a moment for the U.S. and Israel to open doors. Should Hamas choose further confrontation, the door can always be slammed shut.

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