President George W. Bush's plan to send 20,000 more troops to Iraq is a gamble because US forces will be taking on a two-front challenge: fighting Shi'ite militias, possibly in the narrow streets of their Baghdad stronghold, while chasing elusive Sunni gunmen and suicide bombers. The US military has tried this before, though only on a limited basis, in an offensive in the Iraqi capital in the second half of 2006. The operation ended in failure. The bloodshed only increased. Sunni Arab insurgents have easily evaded American offensives in the past. When US troops have seized one area, Sunni fighters have simply moved their suicide bombings and other attacks elsewhere. They've returned once the Americans leave. Shi'ite militias, blamed for the widespread killings of Sunnis that have pushed the country close to civil war, could prove even tougher to control - particularly the most feared militia, the Mahdi Army, led by cleric Muqtada al-Sadr. The Mahdi Army's stronghold is Sadr City, a Baghdad slum of some 2 million Shiites who overwhelmingly support the fighters, seen as their protectors against Sunni gunmen. If US troops decide to confront al-Sadr's militia in Sadr City, American forces will find themselves fighting their way through a maze of narrow urban streets crowded with civilians. Moreover, al-Sadr is a powerful political player in the coalition of US-allied Prime Minister Nouri al-Maliki. Al-Sadr's support was crucial to al-Maliki's election, and the prime minister has so far resisted using Iraqi forces in any offensive against him, to the frustration of US officials. US senators who have been briefed on Bush's plans say he intends to deploy 20,000 more troops, backed by Iraqi security forces, in Baghdad and the western province of Anbar, the heartland of Sunni fighters. The aim of this so-called "surge" is to impose order, giving the Iraq government the breathing room it needs to make progress on both the political and economic fronts. The US will reportedly press the Iraqis to meet a series of benchmarks for steps in reconciling the country's warring Shi'ite and Sunni communities. Success is by no means guaranteed, some analysts say. "This will be a high risk operation," said Anthony Cordesman, a security analyst for the US Center for Strategic and International Studies. "The chances of success are probably somewhat less than even, but the costs of failure are so immense that you have to make the attempt. "You have about a year to deal with the political crisis, the sectarian differences," he said. "The military strategy cannot work without that, but then the political strategy cannot work without (progress on) the security issue." Throughout the political and military campaign, the United States will have to keep al-Maliki's government on board - despite increasing tensions between the two. Al-Maliki bristled at international criticism of his decision to push ahead with the Dec. 30 execution of Saddam Hussein, which many believe has further inflamed Sunni-Shiite tensions. US officials had pressed him to postpone Saddam's hanging. The most recent US attempt to bring peace to the streets of Baghdad illustrates the difficulty that any surge of American troops will face. In late July, the US military launched Operation Together Forward, bringing 14,000 extra troops into Baghdad. The troops went from neighborhood to neighborhood, searching for weapons and battling insurgents and militiamen. As each district was swept, the military brought in cash, employing Iraqis to clean up the streets and funding projects to promote economic development. Still, the bloodshed only increased, with more mass kidnappings, insurgent attacks and bodies - the victims of sectarian killings - found dumped in the capital each day. The Iraqi Interior Ministry reported that the numbers of civilian deaths rose steadily month by month, from 1,062 in July to more than 1,900 in December. By late October, the operation had largely ended. Several weeks later, Sunni insurgents carried out their deadliest attack yet, killing 215 Shi'ites in a string of bombings in the capital. Operation Together Forward targeted both Sunni insurgents and Shi'ite militias. But while Sadr City was on the list of neighborhoods to be pacified, US troops never swept the Shi'ite stronghold - in part because al-Maliki refused to approve what could have been an explosive incursion. For the new plan to work, Cordesman said, US and Iraqi officials will have to make progress on all of Iraq's political, economic and military problems at once - improving security and living standards, and somehow reconciling the country's bitterly divided religious and other factions. "You can't achieve everything all at once, but you have to make progress in all these areas," he said. "It would be a counterinsurgency approach that mixes dollars and bullets. ... You go out, you try, you find out what works. They're going to have to innovate and experiment."