Through its oversight, authorization, and appropriations roles, Congress will likely play a central role in helping implement any decision to increase U.S. troop levels in Afghanistan, as well as any change in their mission. Beyond committing additional forces to Afghanistan, Congress will also likely need to address how sending additional forces to Afghanistan will affect the overall readiness of the U.S. military, the availability of forces in the event of a crisis elsewhere, and what additional resources will be required to support this potential long-term commitment of additional U.S. troops. Today, the Afghanistan War—approaching 16 years—is the longest armed conflict in U.S. history. U.S. military forces entered Afghanistan in late 2001 in response to the September 11, 2001, terrorist attacks (see Figure 1 ). The United States and allies initially drove the Taliban from power and largely destroyed al Qaeda's ability to plan and execute terrorist attacks from Afghanistan. By October 2006, the United States and NATO had assumed responsibility for security across the whole of Afghanistan. In September 2008, President Bush deployed an extra 4,500 U.S. troops as part of what was described as a "quiet surge." In February 2009, the United States announced the deployment of 17,000 additional troops and NATO pledged to increase its military commitment to Afghanistan. In March 2009, President Obama decided to deploy an additional 4,000 personnel to train and advise the Afghan military and police, as well as support the development of Afghan government agencies. In December 2009, President Obama increased U.S. troop numbers by 30,000—bringing the total of U.S. troops to 100,000—and announced that the United States would begin withdrawing its forces by 2011. At the NATO Summit in Lisbon, Portugal, in November 2010, NATO agreed to hand control of security to Afghan forces by the end of 2014. In December of 2014, NATO formally ended its combat mission in Afghanistan and handed it over to Afghan forces. In January 2015, the NATO-led mission "Resolute Support" commenced, with the separate mission to train and advise and assist Afghan security forces. In October 2015, President Obama announced that 9,800 U.S. troops would remain in Afghanistan until the end of 2016, which differed from an earlier pledge to pull out all but about 1,000 U.S. troops from Afghanistan. In July 2016, at the NATO Warsaw Summit, in light of what he called "a precarious security situation," President Obama said that 8,400 U.S. troops would remain in Afghanistan. Also at the Summit, NATO agreed to maintain its troop levels and funding until 2020. Since the late 2001 invasion of Afghanistan, coalition troops have undertaken three basic missions, often simultaneously. The first mission, characterized as counterterrorism, primarily revolves around U.S. efforts to kill al Qaeda terrorists and destroy their networks. It is a mission that continues today. The second mission—conducted unilaterally or in conjunction with allied and Afghan forces—involved direct combat against Taliban insurgents attempting to reassert their control over Afghanistan. As previously noted, this mission ended for U.S. and NATO forces in December 2014. The third mission, which began early in the campaign and is the focus of the Resolute Support Mission (RSM) today, primarily involves working with allies to train and advise the Afghan military and police, as well as support the development of Afghan government agencies such as, for example, the Ministry of Defense. As part of RSM, U.S. advisors are not supposed to participate in direct combat unless it is in self-defense, but under certain circumstances, U.S. air support may be provided to Afghan security forces. Recent press articles suggest that the Administration is considering proposals to deploy additional ground forces to Afghanistan. These forces would likely be part of the Resolute Support Mission, the ongoing NATO mission to train and support Afghan security forces. In testimony before the Senate Armed Services Committee on February 9, 2017, General John Nicholson, Commander U.S. Forces–Afghanistan noted, based on a mission review, that he had adequate forces for the U.S. counterterrorism mission but there was "a shortfall of a few thousand troops" for RSM if what he described as a "stalemate" with the Taliban-led insurgency were to be broken. Recently, in April, about 300 U.S. Marines were deployed to Helmand Province to advise and assist Afghan forces in the region. The current Pentagon request reportedly under consideration, which requires presidential approval, is said to call for expanding the U.S. military role as part of a broader effort to compel the resurgent Taliban back to the negotiating table. As part of this expanding role, the Pentagon is reportedly considering deploying an additional 3,000 to 5,000 U.S. troops to Afghanistan, as well as asking for greater latitude in setting overall troop levels, determining how they might be used, at what level advisors might be assigned (battalion or company level), and providing military commanders greater authority for employing airstrikes. Since the post-9/11 invasion of Afghanistan, the United States and its allies have pursued a variety of different strategic objectives. Within the military campaign alone, those objectives are, at times, in tension with each other. The initial goals for Afghanistan included the elimination of al Qaeda, and in the process, overthrowing the Taliban and installing a new, legitimate government in Kabul. Over time, however, these goals gave way to the more ambitious project of assisting the Afghan government as it extended its reach across the country—an inherently political undertaking—utilizing both civilian and military means to do so. This has led to considerable debate in Washington and around the world about the overall objectives for U.S. and international engagement there. Successive U.S. administrations have sought to synchronize the counterterrorism and nation-building missions, arguing that their operations are mutually reinforcing. In practice, however, their respective operations sometimes undermine each other. As one scholar notes , "the United States might be able to maintain an open-ended military presence in Afghanistan or stabilize the country, but not both." A recent example of these campaign tensions is the use of a GBU 43/B Massive Ordnance Air Blast (MOAB) munition to strike a cave complex used by the Islamic State affiliate in Afghanistan known as Islamic State- Khorasan Province (ISIL-K or ISKP). General Nicholson maintains the MOAB was the "right weapon against the right target," and that it dealt a devastating blow to ISKP. Other observers, however, note while General Nicholson is probably right about the tactical efficacy of the weapon, it has created an opportunity for opposition forces to foment further dissatisfaction among Afghans toward both their legitimate government in Kabul, as well as a continued U.S. presence in the country. Former Afghan President Karzai, for example, described the attack as the use of a weapon of mass destruction by the United States on Afghan soil, and an "immense atrocity against the Afghan people." The use of the MOAB, in his judgment, should be a clear signal to the Afghan people that they should stop the United States. In addition to the issue of different campaign elements sometimes working at cross-purposes, some observers question the overall feasibility of achieving stability in Afghanistan. According to the Special Inspector General for Afghan Reconstruction, territory controlled by the Afghan government has receded by nearly 12% since November 2015, to approximately 60% of the country today. Complicating matters, U.S. commanders and other observers have noted other actors, such as Russia, might be supplying Taliban fighters, frustrating U.S. efforts on the ground. Accordingly, most scholars and practitioners note that the military elements of the campaign are necessary, but not sufficient, to deliver lasting stability in the region. In the first instance, building the conditions that might lead to a lasting peace settlement with the Taliban—the main opposing force to the Afghan government—is generally thought to require a broader strategy that addresses political, diplomatic, development, and governance building challenges. To date, the United States has yet to develop and implement a joint strategy with the Afghans for bringing the war to a successful conclusion. Second, as some note, the U.S. focus on countering terror threats such as those posed by Al Qaeda and the Islamic State rather than the Taliban has created a mismatch between Afghan and coalition objectives; the Afghan government itself is far more concerned with the Taliban. Finally, most scholars note the need to deal with Pakistan, which is generally believed to conduct activities that undermine U.S. and Afghan advances. In all of these respects, it is generally unclear how additional U.S. forces will help resolve these underlying campaign issues. Others take the view that despite these difficulties, allowing Afghanistan to once again descend into chaos would ultimately harm U.S. national security. Such a vacuum might enable terrorist groups—including, but not limited to al Qaeda and the Islamic State—to plan and launch attacks against the United States and its allies. An influx of additional forces might therefore be better able to monitor, if not manage, terrorist groups and other threats using Afghanistan and the region as a safe haven. In testimony, General Nicholson noted he had adequate forces for the U.S. counterterrorism mission but there was "a shortfall of a few thousand troops" for RSM. He suggested additional tr oops were needed to bolster the training and advising of Afghan units, increase advisory efforts across Afghan ministries, and provide more advisory capacity at brigade level. General Nicholson suggested the additional troops could also be provided by NATO partners (more below). DOD's reported current request supposedly will give military officials greater latitude in determining how U.S. troops may be employed, which suggests they could be used not only in ongoing counterterrorism operations but in a combat role as well.