President Obama's Middle East speech yesterday lasted for about 45 minutes, but one solitary line has generated a disproportionate amount of coverage and controversy: "The borders of Israel and Palestine should be based on the 1967 lines with mutually agreed swaps." And on Friday, Israeli Prime Minister Benjamin Netanyahu, for a second timerejected the 1967 borders as "indefensible" during a meeting that he and President Obama just concluded in the Oval Office (pictured above). Why has Obama's 17-word peace proposal proved to be such a lightning rod?

To really unpack this sentence, we need to examine two terms: "1967 lines" and "mutually agreed swaps."

1967 Lines: These refer to the armistice lines from before the Six Day War, when Israel captured the Gaza Strip from Egypt and the West Bank and East Jerusalem from Jordan, among other land, expanding its territory beyond the "Green Line" borders delineated by a 1949 armistice between Israel and its Arab neighbors. The map on the right is from Israel's Ministry of Foreign Affairs (you can visit their site for a larger version). After the war the Israelis, Reuters explains, occupied Gaza and the West Bank militarily and allowed Jews to build settlements in both territories. In 2005 former Israeli Prime Minister Ariel Sharon withdrew troops and settlers from Gaza, which is now controlled by the Palestinian Islamist group Hamas. Hamas rejects Israel's right to exist--Green Line or no Green Line--while Fatah, which rules the West Bank and just signed a reconciliation deal with Hamas, wants a Palestinian state spanning Gaza and the West Bank with East Jerusalem as its capital (in other words, 1967 lines). 

Mutually Agreed Swaps: According to The New York Times, Obama is saying that Israelis and Palestinians would swap territory on either side of the border to account for Jewish settlements in the West Bank and Jewish neighborhoods in East Jerusalem (there are now over 500,000 Jews in the West Bank and East Jerusalem, according to Reuters). In his speech, Obama admitted that he didn't have a solution to propose for two "wrenching and emotional" issues: the division of Jerusalem, which both Israelis and Palestinians view as their capital, and the right of Palestinian refugees or their descendants to return to Israel.

Which brings us to our next question: Why did Obama's comments irk Netanyahu so much? The Israeli prime minister claims an Israeli military withdrawal to pre-1967 borders would render Israel unable to defend itself, despite Obama's assurances in his speech that any deal to create a "non-militarized" Palestinian state would include measures to prevent "a resurgence of terrorism" and "provide effective border security." During his meeting with Obama on Friday, Netanyahu said that between 1948 and 1967, Israel's borders were "boundaries of repeated wars," not "boundaries of peace."

Yet even if Obama's proposal makes Netanyahu worry about Israel's ability to defend itself, it's not as if Obama's  invocation of 1967 borders is groundbreaking. While the Times notes that Obama's land swaps concept is somewhat novel and Israeli diplomat Alon Pinkas adds that the U.S. has never before suggested establishing a Palestinian state before Jerusalem and the refugees issue is resolved, the 1967 borders have long been considered the probable contours of an Israeli-Palestinian peace agreement. 

So, once again, why did Obama's discussion of a well-trodden framework for peace make Netanyahu so angry? Here are the leading theories.

Policy Shift: Netanyahu may have interpreted Obama's words as a major U.S. policy shift. As The Washington Post's Glenn Kessler writes, Obama mentioned the '67 boundaries not as a "Palestinian goal" but rather as "U.S. policy." Yet other analysts, like The Atlantic's Jeffrey Goldberg, argue that Obama's language doesn't signal a break in U.S. policy.

Negotiating Leverage: Jake Tapper at ABC News steps in with an alternative explanation. Netanyahu's issue, he writes, isn't so much the '67 borders themselves "but rather the idea that [Obama's] position essentially gives the Palestinians an achievement at the bargaining table without having conceded anything in return." Now, Tapper explains, "the Palestinians will be able to come to any future negotiation with those borders as the 'American position.'"

You can watch today's press conference between the two leaders here: