Clashes, rocket fire, and threats of escalation are challenging the five-month-old cease-fire between Hamas and Israel in Gaza. In the past two weeks, Israeli forces have reportedly killed 17 Palestinian fighters, while militant groups in Gaza have fired more than 140 rockets into Israel. Despite the benefits of the cease-fire - for Israel, the end of cyclical clashes, rocket attacks and civilian casualties, and for Hamas, a reprieve from Israel's intense military and economic pressure - there is no guarantee it will hold. As such, it is worth considering how the cease-fire might end, what renewed conflict might look like, and what this means for Israel's long-term confrontation with Hamas. Hamas's military buildup Hamas's military capabilities have grown since 2005 as a consequence of four major developments: Israel's August 2005 disengagement from Gaza, Hamas's 2006 electoral victory, Hamas's June 2007 takeover of Gaza, and the June 2008 cease-fire agreement. The disengagement removed the constant threat of Israeli security forces and reduced the ability of Israeli intelligence services to monitor Hamas and other Palestinian terrorist organizations. Hamas used the period after Israel's withdrawal to expand its forces to the point where it could prevail over Fatah and emerge as the only serious military and political power in Gaza. With Israel gone and Fatah defeated, Hamas gained control of Gaza's quasi-state resources, including the military and intelligence assets and infrastructure once controlled by the Palestinian Authority. This dominance removed any serious internal obstacles to Hamas's military expansion, and the subsequent cease-fire agreement created even better conditions for its military ambitions by freeing the organization from the threat of Israeli incursions. Egypt's failure to secure its side of the border also facilitated Hamas's buildup. Hamas aspires to emulate the military capabilities of Hizbullah, and OC Southern Command Maj.-Gen. Yoav Galant has described a Hizbullah-like military force developing in the Gaza Strip. This force includes territorial commands, organized brigade and battalion formations, specialized units, foreign training, and doctrine, tactics, and weapons suitable for fighting the IDF. According to OC Military Intelligence Maj.-Gen. Amos Yadlin, Hamas's defensive preparations in Gaza are "based on subterranean fortifications, explosive devices, and snipers." In future clashes, the IDF would be confronted with a better organized and trained force with more sophisticated arms, especially antitank weapons, and improved defenses. How the cease-fire could break down Four scenarios could produce a collapse of the cease-fire. First, it could simply unravel without any clear decision or breakpoint. The incidents since November 4 suggest how this might work: A defined threat leads to an Israeli raid and a Palestinian retaliation, producing heightened tensions, increased threat sensitivity, and a lowered reaction threshold - all of which create more military and civilian casualties. Eventually, no one is talking about a cease-fire, or if they are, they are using the past tense. Second, the cease-fire could end by accident, most likely as the result of a Palestinian attack. Unguided rockets or mortars fired on towns and settlements would eventually cause significant Israeli civilian casualties. Even Israeli precision-attacks run the risk of collateral damage, since no targeting process is immune from human error, bad information, or happenstance. Third, even limited, well-planned operations can escalate. The kidnapping of IDF soldiers could lead to hot pursuit, and small-force incursions could become entangled with defenders, leading to expanding confrontations as reinforcements are called in. The current series of clashes started on November 4, when the IDF penetrated 250 meters into Gaza to locate and destroy a Hamas tunnel that was reportedly intended to support kidnapping operations. Hamas militants in Gaza maintain close surveillance of the border, follow IDF movements, and are highly sensitive to even small Israeli incursions. They often attempt to engage raiding forces, as responsiveness to Israeli infiltrations appears to be integral to Hamas military doctrine. The IDF, for its part, seems to have reduced its threshold for responding to threats along the border fence. Fourth, Hamas or Israel may decide that the cease-fire is no longer in its interest and deliberately break or not renew it when it expires on December 19. Although the relative calm has served the interests of both sides until now, it may not do so indefinitely. The political context in which military events could occur is evolving with the change of US administrations, the scheduled January end of Mahmoud Abbas's PA presidency, and the Israeli election in February. These scenarios are not mutually exclusive and do not exhaust the list of possibilities for a collapse of the cease-fire. For instance, actions by Gaza or West Bank terrorist organizations not under Hamas control could generate pressure on Israel to act, thus bringing the IDF into contact with Hamas elements. Post-cease-fire military activity If the cease-fire ends, it may be "business a usual": a series of routine actions by both sides, as typified by the pre-truce period. Then, the Palestinians sniped at Israeli targets, planted bombs, fired rockets, lobbed mortar shells, and occasionally engaged in larger operations such as attacks on crossing points, kidnappings, and terrorist attacks inside Israel. Some of these actions were quite sophisticated, as exemplified by the June 2006 kidnapping of IDF Cpl. Gilad Schalit and the April 2008 attempt to penetrate the Kerem Shalom crossing, which included the use of armored vehicles. Hamas regards such actions as part of its panoply of offensive military actions. For its part, Israel has its own routine and special operations: counter rocket/mortar fire, offensive patrolling and incursions, targeted raids against personnel and facilities, and deep incursions when Palestinian actions exceed tolerable levels. Prime Minister Ehud Olmert and Defense Minister Ehud Barak have publicly raised the possibility of such action if rocket attacks continue. Improved Hamas capabilities will likely translate into tougher engagements for the IDF and increased casualties on both sides. Fiercer fighting and greater casualties create pressures to step up the scale and scope of violence. Even if large-scale incursions by Israel are avoided, the fighting is likely to be qualitatively different. Hamas's 2007 destruction of Fatah in Gaza clearly illustrated the axiom that "power grows out of the barrel of a gun," while the group's agreement to the June 2008 cease-fire showed its tactical flexibility. Although Hamas is under political, economic, and military pressure in the West Bank, it holds untrammeled authority in Gaza. While Hamas's strength rests on a combination of political, religious, social, and economic sources, military power is a sustaining force. Without military power, there is no jihad, no "resistance," no "liberation," and no survival for Hamas in the face of its enemies. Pressure from Israel and the PA in the West Bank probably reinforces Hamas's determination to strengthen its hold on Gaza and to expand its military capabilities. Despite its military improvements and ambitions, Hamas could not stand up to the IDF in an all-out fight. But there is little likelihood of such a direct conflict. In combating its irregular enemies, whether in southern Lebanon or in Gaza, Israel's ability to deploy its full military capacity is always constrained by political factors that shape the amount, intensity and duration of the force that Israel can apply. And as Hamas's capabilities grow, it becomes more difficult for Israel to employ limited force effectively with respect to costs and benefits. While further and more-intense military engagements between Hamas and Israel are not predestined, there is little doubt that Hamas poses an increasingly complex military challenge to the IDF. Jeffrey White is a defense fellow at The Washington Institute, specializing in the military and security affairs of Iraq and the Levant. He is also a former career intelligence officer.