Even though movement from the West Bank is restricted by a security barrier, a few terrorists even now manage to enter Israel. With Area C annexed, identifying the few bad guys seeking to kill Israelis among the 300,000 new Palestinian Israelis from Area C (who would have unfettered access to Israel) would be difficult and costly.

The physical barriers required to prevent residents of Areas A and B from filtering into Area C en route to Israel would be a security nightmare. The perimeter of each of the 169 Palestinian islands would have to be treated as an international border. To separate the annexed land from the islands it encircles, 1,200 miles of new barriers would be required, along with hundreds of security gates that would allow controlled Palestinian movement from one enclave to another or from their enclaves to land of theirs in Area C (where 75 percent of the land is owned by Palestinians). The cost of building such a barrier system would be about $10 billion, and constructing the gates, along with associated security measures, would cost far more.

Palestinians would view Israeli annexation as a game-changer, foreclosing the option of a viable Palestinian state. The Palestinian Authority would collapse, and Israel would have to impose martial law and provide basic services to all Palestinians in the West Bank. Defense Minister Avigdor Lieberman has estimated the annual cost of social security alone for Palestinians at $6 billion. The yearly cost of health, education and other government services could be $5 billion more.

With the collapse of the Palestinian Authority, Palestinian-Israeli security coordination would vanish. Many of the Palestinian troops would turn their weapons on Israelis, and the Palestinian street would most likely explode. This would leave Israel’s military and its domestic security agency, Shin Bet, to take full security responsibility not just in the newly annexed Area C, but also for the millions of Palestinians in Areas A and B, where Palestinian security agencies now operate in close coordination with the Israel Defense Forces.

This, in turn, would necessitate an increase in the I.D.F.’s presence throughout the West Bank; the standing army could not do the job alone and a mobilization of reserves would be required. This, too, would tax the Israeli economy and severely diminish military preparedness for other security threats, most directly from Syria, where Iran seeks to establish a presence, and Lebanon, where Hezbollah has become more experienced at combat.

Arab governments might not be able to ignore potentially violent domestic expressions of outrage at Israel’s actions. Accordingly, Israel’s diplomatic and security relationships with Egypt and Jordan might not survive, and chances for additional relationships would vanish.

Israel’s relationship with the American Jewish community would also be jeopardized, with annexation attempts further alienating large numbers of American Jews and accelerating the alarming trend of Jewish youth distancing themselves from Israel — a trend that undermines a major pillar of Israel’s long-term national security.