Pakistani police have identified the Americans detained earlier this week as, from left: Waqar Hussain Khan, Ramy Zamzam, Umar Farooq, Ahmed Abdulah Minni and Aman Hassan Yemer. The men have not yet been formally arrested or charged.  Americans are now learning what citizens of Indonesia, Saudi Arabia, Spain, the United Kingdom, and other foreign countries have long known: that some of our own can and will go to great lengths to kill their fellow citizens.  Ramy Zamzam and four friends from the Washington, D.C., suburbs were detained in Pakistan in a police raid on a house allegedly tied to a militant group earlier this week. One of the men had recorded a video filled with images of war and declarations that young Muslims must act. The five Americans, students in their 20s, are now being questioned by U.S. and Pakistani authorities.  The revelations about the Alexandria, Va., five come just more than a month after Army Major Nidal Malik Hasan allegedly killed 13 of his fellow soldiers at Fort Hood in Texas. The alleged massacre in turn followed revelations that Najibullah Zazi, an Afghan resident of the U.S., who is accused of planning to blow up several targets in New York. Mr. Zazi pleaded not guilty in September. Mr. Zazi's arrest had followed yet another disturbing, if seemingly more-distant report: America had produced its first suicide bomber, who had blown himself up in Somalia in 2008.  This grim catalogue of plots and attacks suggests that the American homeland's holiday from terrorism is ending.  Soldiers storm the Soldier Readiness Center at the Fort Hood Army post in Killeen, Texas, on Nov. 5 after Maj. Nidal Malik Hasan opened fire in a rampage that killed 13 people.  For years since 9/11, I and other terrorism experts sought to explain why Osama bin Laden and his jihadist followers did not hit the U.S. homeland again, a mystery made all the more profound by the deadly jihadist terrorist attacks in Indonesia, Jordan, Spain, and the U.K., among other lands, to say nothing of constant jihadist strikes in Afghanistan, Iraq, and Pakistan.  I explained America's good fortune by a combination of several factors. Perhaps most important, after 9/11 the United States and its allies hammered al Qaeda. U.S. military and intelligence forces threw Mr. bin Laden, his deputy Ayman al-Zawahiri and other al Qaeda leaders out of Afghanistan, shutting down the training camps there and forcing the leaders to find new homes. Less noticed, but no less important, intelligence and security services around the globe hunted down recruiters, planners, financiers, and other key parts of the network. At home, the FBI and other organizations focused intensely on al Qaeda, making it harder for the terrorists to pass unnoticed.  I also took comfort when I looked across the ocean to Europe, where al Qaeda had cells and networks in several countries. In Europe, a mix of discrimination, suspicion, and alienation have created a subculture where terrorists can thrive. Many European countries have Muslim communities that are large and geographically concentrated (Pakistanis cluster in the U.K., Algerians are mainly in France, and Turks are found in Germany), making them less likely to assimilate and more tied to the politics of their native lands. Making these ties even stronger, the trip home is shorter for Muslim immigrants in Europe than in the United States. Most important, Muslim immigrants are often at best tolerated in their new lands, despite their contributions as soldiers fighting on behalf of colonial powers, or later as workers rebuilding Europe. Many Europeans see immigrants and their descendants as eternal outsiders and oppose granting them citizenship.  Even, or perhaps especially, the second and third generations are at risk, as they are often betwixt their ancestral and new homes but accepted by neither. Frenchmen may reject a Muslim who wears a scarf to assert her religiosity, but when she goes to Algeria she is seen as alien and Western, often not speaking Arabic. At times these second and third generations interact with more recent immigrants from Muslim countries, being energized by the disputes in the Muslim world yet keeping their familiarity with the West and European passports, making them potentially dangerous operatives.  Many European Muslims are poor, alienated and angry, while their American counterparts seem wealthy, educated and integrated. While U.S. support for Israel and intervention in Iraq provoked anger about U.S. policies among some American Muslims, they did not display the raw hatred of the U.S. government or embrace of violence as did some among their religious brethren across the Atlantic. Non-Muslim Americans, for the most part, accept Muslim immigrants as a welcome addition to our country, the current incarnation of their Anglo, Irish, Italian or other immigrant ancestors. Minarets may trouble the Swiss, but in America they would never be banned. In July 2005, al Qaeda proved it could tap into jihadist networks in Europe, killing 56 people, including the four suicide bombers, in attacks on London's public transportation system.  Ironically, the terrorism charges levied against various Americans in the years after 9/11 seemed to confirm how much safer our country was. The FBI would often announce arrests of suspects with sound and fury, but in practice they signified how limited the threat was. Those charged were often common criminals or unskilled dreamers, talking big but with little ability to carry out their schemes. Iyman Faris, convicted of providing material support to al Qaeda, initially sought to cut through the many mammoth cables of the Brooklyn Bridge with a blowtorch, an almost impossible scheme.  Others charged had little or no training, and—just as importantly—they did not seem to know how to get in touch with the al Qaeda core. In the end, the government would often charge them with minor, non-terrorism related, crimes such as fraud or violating their immigration status. I am glad the FBI aggressively went after these individuals, some of whom had quite bloody plans, but the suspects were a far cry from steely professionals like Mohammad Atta, the 9/11 team leader. Indeed, the most deadly post-9/11 plot against the United States—the plot to blow up airplanes en route from the U.K. to the United States, which was disrupted in August 2006—was organized and launched from Europe, suggesting much better al Qaeda's networks are in Europe than in the United States.  Today all these factors are changing, and none for the better.  On Oct. 28, 2008, Shirwa Ahmed became the first American suicide bomber. Mr. Ahmed killed himself in Somalia's civil war on behalf of the Islamist group al Shabaab, elements of which have links to al Qaeda in Pakistan. In 2009, al Qaeda's No. 2 Mr. al-Zawahiri called Shabaab advances in Somalia "a step on the path of victory of Islam, while Shabaab would pledge allegiance to Mr. bin Laden. The group even used Alabama native Omar Hammani, who spoke under the name Abu Mansoor al-Amriki ("the American"), to do a video critique of President Barack Obama's speech in Cairo earlier in the year.  Ahmed was part of two groups of perhaps 20 Somali-Americans who grew up in Minneapolis and became radicalized after the Ethiopian invasion of Somalia in 2006. The Somali-American community from which he came has more in common with the Algerians in the banlieues in Paris than the affluent Arab Muslim community of the U.S. By one estimate 60% of the Somalis in the U.S., a community estimated as high as 200,000 people, live in poverty, and many young men drop out of school and turn to crime.  While the conflict in Somalia may seem distant to most Americans, the U.S. role there is real and growing—and Somali-Americans know it well. The 2008 U.S. airstrike that killed Aden Hashi Ayro, a Shabaab leader, joined Somalia's enemy Ethiopia and the United States in the minds of many Somalis. The result, in Somalia expert Ken Menkhaus' words, was that "fierce levels of anti-Americanism took root among many Somalis at home and abroad." In September 2009, the United States struck again, killing another al Qaeda figure there, Saleh Ali Saleh Nabhan. So far, none of the Somali-Americans who went overseas have planned to return home and attack, but the Shabaab's move toward al Qaeda and the anger at U.S. policy are a disturbing combination.  The Fort Hood shootings, the deadliest terrorist attack on U.S. soil since 9/11, are also troubling, as they may show that an unstable American Muslim might express his anger through political violence. As killing sprees from Virginia Tech to Columbine High School show, it is easy for a disturbed individual to pick up a gun and shoot some, or even many, people. Maj. Hassan was in email contact with a radical religious leader from Yemen, according to officials. The silver lining is that the Army believes the suspect acted alone and without any assistance from other terror groups, a view reflected in their decision to charge him with murder in a military rather than civilian court.  The presence of such connections is why the Sept. 19, 2009, arrest of Najibullah Zazi is so disturbing to homeland defense officials. Following the arrest, Homeland Security Secretary Janet Napolitano declared, "Individuals sympathetic to al Qaeda and its affiliates, as well as those inspired by their ideology, are present in the U.S., and would like to attack the homeland or plot overseas attacks against our interests abroad." Mr. Zazi, a legal Afghan resident of the United States for many years, is charged with planning to bomb several targets in New York. In contrast to Americans arrested in the past who were long on violent dreams and short on skills, Mr. Zazi may pose a graver concern. One allegation should be of overwhelming concern: that Mr. Zazi received training during his August 2008 trip to Pakistan on how to manufacture and use explosives and the links he forged there to the al Qaeda core. If true, it would put Mr. Zazi in a league beyond the would-be terrorists who came before him and would make it clear, in case anyone doubted it, that Mr. bin Laden still has unfinished business with the U.S. Mr. Zazi's lawyers have denied he was plotting any attacks or that he undertook al Qaeda training.  It may be tempting to dismiss Mr. Zazi as a one-off, but there is every indication that the al Qaeda core is reviving after its setback in Afghanistan. Each month the group seems more entrenched in tribal parts of Pakistan. While Islamabad had made fitful efforts to uproot it, some of the jihadist groups the regime nurtured and tolerated to fight India and advance Pakistani interests in Afghanistan have turned against the regime, while others are off the leash. Mr. bin Laden now has close ties to several groups that have tens of thousands of supporters in Pakistan, and his reach is growing to non-tribal parts of the country. In his sanctuary, he and his lieutenants can plan, recruit, issue propaganda, and train the next round. While in 2002 would-be terrorists in the United States had no obvious place to go for training, now even the most casual news-reader, to say nothing of those who trawl jihadist sites that appeal for volunteers, knows to go to Pakistan.  Several steps are necessary to keep our country safe. First, fighting the al Qaeda core in Pakistan should remain at the center of U.S. counterterrorism policy. Having a secure haven is often a make or break issue for terrorist groups, and al Qaeda's growing strength there is a deadly danger. U.S. drone strikes, a program that accelerated near the end of the Bush administration and took off in the first months of Mr. Obama's term, keep al Qaeda off-balance, but they are not a substitute for forcing Pakistan to clean out this haven. We mustn't forget that Mr. Zazi managed to receive training after the drone strikes began in earnest.  Second, we need to consider how American foreign policy can lead to domestic radicalization. Killing an al Qaeda leader in Somalia is a blow to the organization there, but the decision on whether to pull the trigger or not should also factor in the risk of radicalizing an already alienated immigrant group here at home, not just the operational benefit of removing one leader from the organization.  Terrorism suspect Najibullah Zazi is arrested by FBI agents in Aurora, Colo., on Sept. 19.  Third, the FBI and state officials should redouble efforts to know local Muslim communities and gain their trust. Counterterrorism involves not only Predator attacks, but also social services for immigrant communities and courtesy calls to local religious leaders to hear their concerns and assure them that the United States continues to welcome them.  As the allegations about Maj. Hasan, Mr. Zazi, and the Alexandria five suggest, you don't need a large and alienated community in order to have terror threats. But here at least there is good news, for counterterrorism is far more effective when the community is integrated and friendly to the government. Tips from local communities facilitated many of the post-9/11 arrests related to terrorism. And in the case of Mr. Zamzam and his friends, family members in the United States consulted the Council on American-Islamic Relations, or CAIR, which—recognizing the potential danger—put them in touch with the FBI. In contrast to the combative stances of many European Muslims toward their governments, most American Muslims trust their government, and relationships are growing stronger.  To improve this further, government officials should continue and expand outreach efforts to the Muslim American community. From the President's Eid al-Fitr greetings to Muslims down to town councilors swinging by for a local celebration, these gestures are powerful signs of welcome and stand in sharp contrast to the cold shoulder given Muslims in many European towns. And before new counterterrorism measures are announced, officials should consider how they would be perceived in the Muslim community as well as their immediate benefits for intelligence collection or better border security. Growing vigilance against any emerging threat must ensure the Muslim community feels respected and integrated, as this is the best way to make sure the holiday from terrorism does not end.  Daniel Byman is the Director of the Center for Peace and Security Studies at Georgetown University and a Senior Fellow at the Saban Center at Brookings.
The U.S. is facing rising terror threats from its own citizens. What made the country safer after 9/11 is changing, and not for the better, argues Daniel Byman.