The return of the IDF to Gaza marks the end of the first phase of the post-disengagement experiment, less than a year after it began. The two immediate causes are obvious: Palestinian tunnelling from Gaza into Israel, resulting in the killing of two soldiers and the kidnapping of another; and the escalating rocket attacks against Sderot and elsewhere. But the underlying factor, and the more lasting impact, is the rediscovery that as long as the Palestinian war continues, there is not substitute for an Israeli military presence on the ground. The innovative arrangements that accompanied the withdrawal of Israeli military forces from Gaza in August 2005 have all collapsed. Once again, the pessimists turned out to be realists. The transfer of responsibility for preventing the smuggling of weapons and terrorists to Egyptian and European border monitors was a complete failure. The warheads of the Kassams and the anti-tank missiles fired in the recent attack were brought in under their noses - or perhaps their feet. WHEN PRIME minister Ariel Sharon decided to remove the IDF from the 13-kilometer long Philadelphi corridor that separates Gaza from Egypt, many Israelis warned of what might happen. Following detailed negotiations, Israel agreed to allow Egypt to bring in additional forces and weapons (beyond those specified in the 1979 peace treaty) so that they could block the illicit activity. Under the eye of the American-led Multinational Force and Observers, Egypt was supposed to stop the Palestinians from digging tunnels for use in smuggling explosives, terrorists and anti-tank and even anti-aircraft missiles. And the Israeli government's diplomatic advisers argued that by withdrawing its presence completely, the occupation of Gaza would be seen as having ended, as well as responsibility for the welfare of its population. Shortly after this agreement, US Secretary of State Condoleezza Rice brokered a separate security arrangement covering the Rafah crossing. On November 15, 2005, Rice "knocked heads together," and Sharon agreed to allow Palestinian Authority control of this border zone, accompanied by a European Union monitoring mission to insure implementation of the prohibition on the entry of weapons and terrorists. This effort was also a response to the pressures from Javier Solana, the European Union's high representative for the common foreign and security policy, and James Wolfensohn, who was the Quartet's special envoy in Gaza. At the end, Rice triumphantly declared: "This agreement is intended to give the Palestinian people freedom to move, to trade, to live ordinary lives." INDEED, this effort was based on the hope that the Palestinians would adopt these goals and act accordingly. The US and Europe were trying to prop-up the post-Arafat leadership, headed by PA President Mahmoud Abbas, despite its record of decades of corruption and anarchy. President George W. Bush pledged American leadership, "working together with Jordan, Egypt, and others in the international community, to build the capacity and will of Palestinian institutions to fight terrorism, dismantle terrorist organizations and prevent the areas from which Israel has withdrawn from posing a threat that would have to be addressed by any other means..." These hopes went nowhere. As the Israelis left, the Palestinians extended the terrorist infrastructure into many of the greenhouses, schools and structures of the former settlements. A few months later, Hamas took over, pledging to eradicate Israel and escalating the attacks. Nor did the diplomatic gains materialize - Israel is still blamed for the plight of Gazans. AFTER PAYING a high price, Israel has rediscovered the fundamental need for direct control over the border between Gaza and the Egyptian Sinai. This lesson has been learned on many occasions - in 1948, 1967 and 1973 (when Egypt relocated anti-aircraft positions after the American-brokered cease-fire had supposedly frozen their locations). But each time, after paying the price of the previous round, Israel has been forced to resume responsibility. It is still far too early to know how the return to Gaza will end. But even if the kidnapped soldier is released, Israeli forces cannot simply turn around and leave, waiting for the next round in the war. Reoccupying the poor and hate-filled cities would be a mistake, but the days when Palestinians terror cells could simply go from Egypt into Gaza with their weapons are over. Having relied on Egypt and Europe to no avail, Israel now has no choice but to resume direct control over Gaza's borders. The lessons of the failed Gaza disengagement must also be applied to the West Bank, where Israel will need to maintain tight control over the Jordan Valley. Perhaps the next time Israel agrees to transfer border responsibility, it will be after the Palestinians have finally accepted the need prevent attacks from being launched from their side of the border. The writer is director of the Program on Conflict Management at Bar-Ilan University.