LONDON Military targets will grow harder to hit and the risk of deadly errors will increase as Western nations pursue their air campaign over Libya, with no guarantee of stopping Muammar Gaddafi's forces in their tracks.
While three nights of bombing have enforced a no-fly zone and appear to have knocked out surface-to-air missile defences, they have yet to stop Gaddafi loyalists attacking rebel cities.
The overnight crash of a U.S. Air Force F-15E fighter jet with mechanical failure served as a reminder of the risk of mishaps, even though both pilots ejected safely and avoided falling into the hands of Gaddafi's forces.
To keep up the momentum of their bid to enforce a total ceasefire, coalition pilots must go on striking. And the targets they are assigned as time goes by are likely to get riskier in terms of possible civilian casualties, and more controversial in terms of agreed war aims.
NATO's 1999 bombing of Yugoslavia to halt an impending humanitarian catastrophe in Kosovo started out in much the same way. NATO hoped two or three days of bombing would prove it was not bluffing and compel Serbian autocrat Slobodan Milosevic to withdraw his forces from the rebel province.
But the air strikes lasted from March 24 until June 10.
It took 11 weeks, in which NATO ran out of targets, sending pilots back again and again to "bounce the rubble" of military sites they had already destroyed. Civilians were killed in strikes that went wrong, and NATO solidarity was battered.
Belated admissions of regrettable "collateral damage" helped to fuel mounting protests against the war in Western capitals.
RISK OF MISTAKES
Libya's government says dozens of civilians have already been killed, though its claims so far are impossible to verify.
"You can always get 'lucky' with air power -- a strike that kills Gaddafi, for instance," said U.S. Naval War College professor of national security studies Nikolas Gvosdev.
"But increased reliance on air power raises the costs, particularly the chances of collateral damage."
Faulty intelligence or an errant allied missile could destroy a school, a hospital or a mosque, killing dozens.
"The danger is that this undoes much of what was good about the Arab spring," said Rosemary Hollis, professor of Middle Eastern policy studies at London's City University. "The risk is that it is seen as the West again interfering in the way it is always seen to, and simply making matters worse."
In the Kosovo campaign, by the end of Day 3, almost all strategic military targets were destroyed. But the Yugoslav army was still fighting in Kosovo. Phase Two turned to attacks on those ground forces. Then it went on to bridges, factories, and government buildings including army HQ in the capital, Belgrade.
Targets on the original "no strike" list were eventually hit, often after tense disputes among allies desperate to minimise the risk of civilian casualties and to avoid any charge that their aims were in any way malicious.
NATO pilots bombed the Chinese embassy in Belgrade, killed 73 in a Kosovo refugee column and obliterated a rebel camp by mistake. They destroyed the main bridge over the Danube in Serbia's second city, inflaming Serbian patriotism.
Solidarity was strained near breaking point when NATO's supreme commander, U.S. General Wesley Clark, ordered strikes on the office tower of Serbian state television -- the "voice of Milosevic's war machine" -- and civilians were killed.
NO "SURGICAL" WAR
Bombing from high altitude to avoid pilot losses that could cripple public support for the intervention, NATO wasted tonnes of munitions blowing up 'tanks' that turned out to be wooden decoys.
In the end, despite vows not to get into a ground war, preparations for exactly that option got the go-ahead from U.S. President Bill Clinton, who ordered an advance force of Apache helicopter gunships to Kosovo's neighbour, Albania.
Milosevic capitulated after 78 days and it was never used.
But the British commander, General Mike Jackson, a Kosovo veteran, said this week that he "wouldn't put too strong a possibility on Gaddafi voluntarily throwing in the towel".
Gaddafi could switch to guerrilla warfare "which is rather more difficult to deal with using conventional military means, from the air and from the sea", he told Sky News, and "there may have to be action taken against him from within Libya itself".
In the Kosovo campaign, military leaders repeatedly warned political masters that war was an open-ended proposition and could never hope to be clean and "surgical".
The first phase of air defence "suppression" in Yugolsavia would take "a day or two, maybe six", Clark estimated, according to his memoir, Waging Modern War. But plans had to include ground attacks and in the end airpower might not finish the job.
"There was no way we were going to be able to stop Serb paramilitary forces ... going in and murdering civilians," he recalls warning of the limits of airpower.
In Libya, any proven civilian casualties could play into the hands of Gaddafi while his troops continue fighting in cities where his forces are hard to target.
Some analysts already see a possible "stalemate", a word used in the Kosovo context, which then became "quagmire".
That could demolish Arab support for intervention and turn the youthful protest generation in nearby countries against the West -- which already faces suspicions of hypocrisy for doing nothing as other autocratic Gulf states clamp down on protest.
It could strengthen the hand of Islamist militant groups who have so far been struggling to find their place as pro-democracy protests grip North Africa and the Middle East.
With NATO members Turkey and Germany objecting to alliance involvement and Washington preparing to step back from the limelight after its opening salvoes of cruise missiles, France and Britain may end up taking the lead the campaign into a much trickier and unpredictable second phase.
Some analysts say the campaign -- already somewhat vague in its aims -- could run out of control with its masters simply unsure of what their core aim really is. The stated aim is to protect civilians, not to topple Gaddafi, though officials from different countries involved offer different nuances of that.
"Worst case scenario?" said the U.S. Naval War College's Gvosdev. "In the short term, the rebels prove unable to take advantage of allied air power to push Gaddafi back.
"The operation thus spirals out of the limited goals that defence officials have been insisting on and mission creep sets in."
Human Rights Watch estimated that between 489 and 528 Yugoslav civilians were killed in 90 incidents in NATO's Operation Allied Force over Kosovo. Serbia says the total was considerably higher.
The alliance suffered no combat deaths.
But in order to enforce withdrawal of Milosevic's army and its separation from the guerrillas they were fighting, NATO deployed over 60,000 troops into Kosovo in June 1999, and it still has a peacekeeping mission there today.
(Editing by Mark Trevelyan)