President Barack Obama wants Israelis and Palestinians to return to the bargaining table, and he repeated the call Sunday in a speech to Israel supporters. But it seems unlikely this will happen anytime soon — and even if it did, the sides would find a formidable array of obstacles to agreement.

Obama is clearly aware of this, telling the American Israel Public Affairs Committee that "no matter how hard it may be to start meaningful negotiations under the current circumstances, we must acknowledge that a failure to try is not an option."

"Even as we are clear-eyed about the difficult challenges before us ... I hope we do not give up on that vision of peace," he added.

Among these challenges are huge gaps on important issues between Palestinian President Mahmoud Abbas and Israeli Prime Minister Benjamin Netanyahu, a right-winger for whom acquiescence to the very idea of Palestinian independence — popular around the world and now widely accepted in Israel, too — was a major ideological leap.

But even if a more compliant Israeli leadership should return to power, any negotiators would face some daunting obstacles:

BORDERS

Obama made waves with his declaration Thursday that a peace treaty should be "based on the 1967 lines with mutually agreed swaps." It differed only in nuance from previous U.S. positions — a point Obama stressed Sunday — but hearing the principle stated clearly by the U.S. president had been a major Palestinian objective, and it touched a deep nerve in Israel, too.

Netanyahu swiftly declared the 44-year-old lines "indefensible" from a military point of view. And a look at the map shows why: Israel would be about 10 miles (about 15 kilometers) wide at its narrowest point; the West Bank surrounds the Israeli part of Jerusalem on three sides; and, on a clear day, the West Bank's strategic highlands are clearly visible from Tel Aviv, where about a quarter of Israelis live. If there is any chance that a future Palestine could turn hostile, these borders are a challenge.

Are they sacrosanct — or somehow enshrined in international law?

American officials are generally careful to use the word "lines" and not "borders" when referring to the demarcation that lasted from the end of the 1948-49 war after Israel declared independence until the 1967 war when it expanded its territory. That is no coincidence: these are temporary armistice lines between Israel and Jordan in the case of the West Bank, and Israel and Egypt in the case of Gaza. Those two countries captured the areas — previously part of British-ruled Palestine — in that 1948-49 war.

Might Israel keep some of its 1967 booty?

That largely depends on how hard the Palestinians press, and how much leverage they can summon up. U.N. Security Council Resolution 242 of 1967 seemed to leave the door open — calling for "withdrawal of Israeli armed forces from territories occupied in the recent conflict." It avoided use of "the territories" and left everyone to debate whether this meant Israel could keep some areas.

On Sunday, Obama predicted the sides eventually "will negotiate a border that is different than the one that existed on June 4, 1967 ... to account for the changes that have taken place over the last 44 years, including the new demographic realities on the ground." That was a reference to the settlements Israel has built — and one the Palestinians will not appreciate.

Borders were supposed to be the simplest issue in peace talks, yet in a numbing two decades of talking the sides could never quite agree.

Gaza is simple enough, because Israel does not challenge the pre-1967 line and removed its relatively few settlers from the territory in 2005.

But more than a quarter million Israelis live throughout the West Bank now — in addition to a similar number living in the occupied sector of Jerusalem which is adjacent to the West Bank. Most of the settlers live close to the pre-1967 border. That makes it seemingly practical to include them in a redrawn Israel. Obama accepts this idea, but calls for land Israel receives to be swapped for unpopulated parts of Israel adjacent to the West Bank.

But must the swaps be equal in size? And how much land can they involve?

Obama did not specify — and the second question is critical, because there are at least two major settlements — Ariel and Maale Adumim — that have tens of thousands of residents and are deep enough inside to disrupt things badly for the Palestinians. Palestinians know that going around Maaleh Adumim — if it were part of Israel — would turn a 15-mile (25-kilometer) drive from Ramallah to Bethlehem, major West Bank centers, into a circuitous ordeal. Israelis have tended to assume creative cartography will finesse the issue. But if the swaps will be tiny, that probably means these two settlements would be evacuated.

The failure to agree even on borders suggests the issue is more complicated than it appears. For the Palestinians, getting even all of the West Bank and Gaza means accepting the loss of almost four-fifths of historic Palestine — and they're in no mood to give up yet more. And the Israelis — looking at the current map, and not so much at history — are basically uncomfortable with the smallness of their state.

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JERUSALEM

Dividing Jerusalem is even tougher than negotiating the West Bank borders.

The walled Old City, an area of less than a square kilometer (mile), houses some of the world's holiest sites for Jews, Muslims and Christians. Neither Israel nor Palestine could easily give it up to the other. Before 1967, it was part of Jordan — but a 1947 U.N. partition plan for Palestine called for internationalization of a wide area around the entire city after a British departure.

During past peace talks, the sides spoke of each controlling its "own" holy sites — but were not known to have reached a detailed understanding of how two states could divide between them an ancient enclave full of warrens and alleyways, ancient ruins and underground tunnels and excavations. Would there be a border? Who would be in charge of security? At one point there was even talk of the most explosive site — known as the Temple Mount to Jews, and Haram as-Sharif to Muslims — being placed under "divine sovereignty" to sidestep the problem.

But even beyond the Old City, Jerusalem's current demographics defy a division anywhere near as clean as, for example, the wall that once divided East and West Berlin.

After 1967, Israel expanded the municipal borders into the West Bank. Over the years it has ringed the Arab-populated part of the city with Jewish neighborhoods. The Palestinians call them "settlements" no different from those in the West Bank, and indeed, some have the appearance of distinct hilltop communities. Some 200,000 Jews now live in such developments in the occupied area of the city, alongside about 300,000 Palestinians and 300,000 Jews in the western part of Jerusalem.

The sides have discussed the principle of each keeping those areas of the city where its people live — but again, without much detail. On the ground, such a division would yield an astoundingly kaleidoscopic jumble, with islands of Jews surrounded by Palestinian areas and vice versa. A light railway planned for the city could end up crossing several borders a minute.

Jerusalem's mayor, Nir Barkat, put aside arguments about national rights and religious holy sites and argued plainly, in a meeting with foreign media this month, that a division of the city was no longer a practical possibility.

Yet to the Palestinians, Jerusalem is the heart of their country, and it is difficult to see them accepting a merely face-saving formula — such as access to, or some sovereignty over, their holy sites. Peace probably requires doing what Barkat argues is impossible.

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REFUGEES

The Palestinians have always demanded a "right of return" for Palestinian refugees and their millions of descendants to their families' previous homes in Israel — even though in most cases the homes, and in some even the villages, no longer exist.

For Israelis across the political spectrum this is a non-starter. The main reason they did not annex the West Bank and Gaza — and the reason why many are willing to part with such strategic territories — can be boiled down to a desire to ensure their Jewish majority.

On occasion, Palestinian officials would hint that a formula was possible that would satisfy everyone — perhaps, for example, with the right declared in principle but implemented only for a small number. A 2002 peace initiative by the Arab League made only indirect reference to the refugees, giving some Israelis hope.

But the deep Palestinian yearning is still there, seeming to grow stronger with each generation that grows up disenfranchised in countries such as Syria and Lebanon. Youth who have never seen their ancestral land carry keys to vanished family homes. Earlier this month, thousands risked their lives trying to breach Israel's borders, and several were killed by bullets fired from rattled Israeli troops.

At the White House on Friday, Netanyahu said the Palestinians must be told clearly that a return is "not going to happen." With this statement, the often divisive Netanyahu spoke for the vast majority of Israelis. In his speech a day earlier, Obama had sidestepped the vexing issue.

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According to the original timetable of the 1990s, a comprehensive deal ending a century of conflict was to be reached by May 1999. That never happened, and still seems far from imminent today. What, then, are the alternatives?

For one thing, the Palestinians say they will ask the United Nations for recognition of a state along the pre-1967 lines in September. Obama is trying to dissuade them.

If the Palestinians proceed, the gambit promises to be messy. Since the United States can veto any move in the Security Council, the Palestinians' bid would likely pass only in the General Assembly that has declarative but not practical powers. Still, such a recognition could spark other moves, including economic boycotts against Israel and mass public protest in the West Bank. Israel does not take it lightly.

A host of other scenarios, in the short and long term, could possibly unfold:

— An interim deal: Israel would probably jump at a plan establishing a Palestinian state on most of the West Bank and all of Gaza, leaving Jerusalem and the other issues for later, and not requiring the Palestinians to forswear all future claims. The Palestinians, fearing the temporary will become permanent, reject this out of hand — but world pressure might change this.

— A unilateral pullout: In 2006, Israeli Prime Minister Ehud Olmert said he would unilaterally pull out of most of the West Bank, essentially implementing the interim scenario without Palestinian agreement. The Gaza precedent now works against this in Israeli public opinion: Israel pulled out of the coastal strip, Hamas militants soon seized it, and the area has been used as a launching pad for rockets against Israel. But some variant of unilateral pullout may regain favor, especially if Israel faces mass Palestinian unrest that gets out of hand. Some speak of removing some settlers — but keeping the army in place for now.

— Outside intervention: It seems far-fetched today, but some Palestinians speak of asking the U.N. for a "trusteeship" over their areas, not unlike the British "mandate" over all of Palestine conferred by the League of Nations in 1922. Israel would find it tough to rebuff. It seems unlikely except as a very last resort, because it would probably require the dismantling of the Palestinian Authority and a sort of admission that the Palestinians aren't ready for independence.

— A binational state: Few on either side say they want this today. But if Israel cannot extricate itself from the West Bank, in the long run it would face pressure to give the Palestinians the right to vote, much as South Africa did when ending white minority rule. The Palestinians are already about half the population in Israel plus the West Bank and Gaza — and barring massive Jewish immigration they will very likely become the majority through their faster birthrate. In an irony of history, Jewish nationalism — in bonding Israel to the areas it conquered in 1967 — would have helped bring down the Jewish nation-state.