Listening to the US president Barack Obama and his European colleagues setting out the limits of their military engagement in Libya, its worth remembering the famous warning by Prussian General Helmut von Moltke that no battle plan survives contact with the enemy. As US cruise missiles destroyed Libyan air defence batteries and French fighters took out four tanks attacking the rebel stronghold of Benghazi, Mr Obama told the world that he had no choice but to launch limited military action to prevent Colonel Muammar Gaddafi realising his brutal intentions. But Mr Obamas key message was aimed at Americans: We will not - I repeat - we will not deploy any US troops on the ground. The New York Times reports that Mr Obama had also insisted to his aides that US military involvement must be over within days, not weeks. Following a summit in Paris of the nations involved in the military campaign authorised by last weeks UN Security Council resolution 1973, the French president Nicolas Sarkozy insisted that regime change was not the goal of the air campaign, and that the door of international diplomacy would open to Colonel Gaddafi once he ended his attacks on rebels and their supporters. Western leaders have made no secret that they want Colonel Gaddafi out, with Mr Obama, Mr Sarkozy and the British prime minister David Cameron all having declared unambiguously that the Libyan strongman had lost his legitimacy. But their military campaign was adopted as an emergency response to the intolerable probability that without foreign intervention, Colonel Gaddafi could sack the rebel capital of Benghazi and exact vicious reprisals on an epic scale. Optimists in western corridors of power hope that the shock and awe effect of their air campaign prompts the regimes collapse amid mass defections. But optimism is the opiate of the interventionists, and western leaders would do well to prepare for some nastier contingencies. Its almost inevitable that mistakes by coalition pilots result in civilian casualties - a scenario Colonel Gaddafi will work hard to engineer by the placement of his military resources, and whose probability was underscored on Saturday when rebel fighters in Benghazi appeared to have downed a fighter jet piloted by one of their own. More importantly, even when in an aggressive fashion, air power rarely succeeds on its own in dislodging an enemy. UNSC Resolution 1973 allows the use of force only to protect civilians, however, and not to provide air support to a rebel military advance. The Security Council has also forbidden governments from arming the rebel forces, as Egypts military is reported to be doing already, albeit discreetly. The resolution, instead, ties the protection of civilians to the demand for a ceasefire (by all sides) and a negotiated political solution. That gives Colonel Gaddafis regime considerable wiggle room. Although Tripoli initially announced its acceptance of the ceasefire, it never stopped its advances on rebel strongholds. Still, a truce remains an option at any time, as Mr Sarkozy himself conceded, opening the door to resumed diplomacy that could become increasingly messy. Colonel Gaddafi continues to command a degree of popular support, and is relying on the passion of his supporters, infused by the foreign intervention with national fervour, to even up the odds by starting to hand out weapons, hoping to fight the battle on terms that negate the effectiveness of a western intervention confined to bombing heavy weaponry from the air. The use of air power to destroy armour and artillery on the ground does even up the odds that had recently seen the rebels forced onto the back foot, and leaves the regimes forces vulnerable in the east, where most of the regular army has joined the rebel side. The rebels also have a numerical advantage in Benghazi and other eastern cities. But the fight could become increasingly bloody at close quarters in the days ahead, as Colonel Gaddafi makes the most of the reluctance of his foreign adversaries to commit ground forces. Mr Obamas days could very easily stretch to weeks, or even longer. Indeed, if the regime survives the shock and awe of the initial foreign intervention, the western powers that are running the campaign will find themselves locked in to a longer and more complex war than they intended. Given the fate that awaits him if he quits, Colonel Gaddafi has plenty of incentive to raise the stakes and hope that limited political resolve forces his adversaries to fold. And having armed his most committed supporters, as the rebels have done, Colonel Gaddafi has helped ensure that even in the best-case outcome, foreign troops may be needed on the ground to keep any fragile peace that emerges, while a new Libyan state is created on the ruins of Gaddafis personality cult regime. But no one wanted to talk about end games - either a strategy for removing Colonel Gaddafi, or what would follow his ouster - last week as the tyrants forces bore down on Benghazi. This war was forced by an urgent need to do something to stop Colonel Gaddafi crushing the rebellion and butchering tens of thousands of civilians. The realist camp in the Obama Administration, led by the defence secretary Robert Gates and the national security adviser Tom Donilon, were focused on strategy, consequence, and end-game, and on that basis warning Mr Obama against getting involved in a conflict whose outcome was not vital to US national interests. But Colonel Gaddafis blitzkrieg tipped the scale in favour of humanitarian military intervention, as advocated by the US secretary of State Hillary Clinton, her top adviser Samantha Power, and Susan Rice, the US ambassador to the UN. Despite Mr Obamas statements on limits of the engagement, the realists know that wishful thinking will count for little. The urgency of responding to Colonel Gaddafis march on Benghazi with murderous intent had prompted Western leaders to set aside questions of an endgame in launching military action. Interventions that are not guided by a strategy, but by good intentions, dont always lead to happy outcomes. The enemy usually has some ideas of his own about how the war will be fought.