Kerry's next move in the Middle East Baseball Hall of Famer Yogi Berra once said famously: "When you come to a fork in the road, take it." Secretary of State John F. Kerry's peace process diplomacy is a living testament to the paradoxical wisdom of that quote. On the one hand, Kerry's efforts have been marked by an unusual degree of deftness and political smarts. He finessed the two basic steps that have foiled his recent predecessors: the effort simply to start negotiations, and the search for confidence-building measures. Rather, he patiently built a strategic construct designed to make it very difficult for Israelis or Palestinians to avoid resuming, at least, peace engagement. First, he brought Arab foreign ministers together in Washington to reaffirm the important 2002 Arab peace initiative and induced them to add some flexibility on the delicate issue of exact borders. Second, he underscored the importance of bringing into being a viable Palestinian state by supporting a World Economic Forum initiative to invest $4 billion in Palestinian economic development. Third, he supported a far-reaching but often neglected component of peacemaking - people-to-people peace activities - in the form of an Israeli-Palestinian joint business initiative. Fourth, through quiet diplomacy, not public pronouncements, he tried to achieve a slowdown in Israeli settlement activity, an effort that has apparently met with less than success. And finally, he was helped by the appointment of retired U.S. Marine Gen. John R. Allen, the former top commander in Afghanistan, to address Israel's security concerns, a sign of the importance the United States attaches to those concerns. These tactical moves allowed Kerry to arrange an initial meeting Sunday night and direct talks Monday between senior Palestinians and Israeli negotiators in Washington to try to nail down agreement to move to final status negotiations. Although these talks have been greeted with a sarcastic "it's only talks about talks," it does represent a baby step forward in the effort to resume serious, substantive peace negotiations. But Kerry's diplomacy has also left open the option for both parties to take the other fork in the road: to say no. It is clear that, even with Kerry's mediation, the two sides are not in agreement on the starting points, the "terms of reference" for those negotiations. What, then, do they plan to talk about? The gaps are presently quite wide in all of the contentious core issues. Moreover, Israel insists that it will not start the talks on borders on the basis of the 1967 lines, and it demands that Palestinians accept Israel as a Jewish state. Palestinians say they won't negotiate borders unless 1967 is the starting point, and they demand a freeze on settlements and insist on the release of dozens of long-term prisoners. It is hard to see how Kerry can square these circles without defining more specifically the pathway forward - by laying out an American view of the parameters of a final peace settlement. Kerry needs to provide a comprehensive picture of the core issues that will narrow the points of disagreement, point the way toward bridging those disagreements and include a design for the architecture of the negotiating process. Kerry need not ask the two sides to accept these parameters - they will reflect U.S. policy - but he should expect them to use the parameters as the terms of reference for their negotiations. Second, Kerry should maintain a very active American role to help the parties to bridge divergences. For example, he should seek mutual "deposits" - contingent agreements "held" by the U.S. that can be applied as long as the terms are met - to address territorial endgame and other core issues. Both sides will challenge bold moves such as these. Both will find fault with some aspects of the parameters. Each will blame the other for intransigence. The usual "spoilers" - terrorists and radical settlers - will try to scuttle the process. This is where Kerry and President Obama will need to show their mettle. The United States will need to make it clear that "no" is not an acceptable answer and be ready to use American diplomatic and other pressure to ensure that both sides know the U.S. is determined to succeed. Polls conducted in May in Israel indicate that if Prime Minister Benjamin Netanyahu would adopt the Arab peace initiative, which means an end to Israel's conflict with the Arab world, a 69% majority would support him, comprising 40.8% who would positively support and 28.4% who would not oppose. Only 17.5% would strongly oppose. Kerry must be agile, creative and determined in the tactical moves that lie ahead. Steps, including constructive unilateral ones, that create a reality of "two states for two people" should be gradually implemented even before an agreement is reached. Strong U.S. parameters, and U.S. insistence that the parties negotiate on the basis of these parameters, along with equal U.S. determination to exact consequences for the failure of the parties to engage is the best way forward. Indeed, Kerry must provide the diplomatic GPS to ensure that Israelis and Palestinians in fact choose the right fork in the road ahead. Daniel Kurtzer, former U.S. ambassador to Israel and Egypt, teaches at Princeton University's Woodrow Wilson School of Public and International Affairs. He edited the book "Pathways to Peace: America and the Arab-Israeli Conflict." Gilead Sher, co-chairman of the Israeli movement Blue White Future, is a senior research fellow at the Institute for National Security Studies in Tel Aviv and a former Israeli chief negotiator.