Jeh C. Johnson had a clear view from his law firm’s 28th-floor Manhattan office as a jet hit the second World Trade Center tower on the morning of Sept. 11, 2001.

“The images of smoke and destruction against the backdrop of a clear, beautiful blue sky are burned into my memory,” he said at an American Bar Association breakfast four years ago.

Johnson, who turned 44 on what he called “one of the darkest single days in American history,” is now poised to take the helm of the government agency created a decade ago to help ensure that such an attack never happens again.

On Friday, President Obama nominated Johnson, who from 2009 to 2012 served as the Pentagon’s general counsel, to head the Department of Homeland Security. “From the moment I took office, Jeh was an absolutely critical member of my national security team, and he demonstrated again and again the qualities that will make him a strong secretary of homeland security,” said Obama, for whom Johnson was a foreign-policy adviser and fundraiser during his 2008 presidential campaign.

“Jeh has a deep understanding of the threats and challenges facing the United States,” Obama said. “As the Pentagon’s top lawyer, he helped design and implement many of the policies that have kept our country safe, including our success in dismantling the core of al-Qaeda” in Pakistan.

Jeh Johnson was nominated Friday to be the next Secretary of Homeland Security. But who is he? And how do you pronounce his name? (The Washington Post)

Cobbled together from 22 different agencies, DHS represents the single-most ambitious transformation of U.S. government in more than half a century. Johnson, who is expected to be confirmed without much difficulty, will face the challenge of leading that sprawling 240,000-person organization and continuing his predecessors’ work of forging a unified culture out of entities as disparate as Immigration and Customs Enforcement, the Coast Guard and the Transportation Security Administration.

“Sometimes it can take a whole generation for a bureaucracy the size of DHS to gel, and that is a really difficult challenge,” said Daniel Benjamin, a former State Department counterterrorism coordinator.

If approved by the Senate, Johnson faces the immediate challenge of replenishing a department in which 40 percent of senior leadership positions are vacant or have an “acting” placeholder.

Although he lacks executive experience, Johnson, a personable native New Yorker, has dealt with complex organizations as the Defense Department’s top lawyer, supporters say.

When he was the Pentagon general counsel, Johnson had a hand in nearly every major policy issue facing the department and its 3 million employees, “the largest organization on the planet,” said former defense secretary Robert M. Gates, who worked closely with Johnson.

In particular, Gates pointed to Johnson’s leadership in the 2010 review process that led to the repeal of the military’s “don’t ask, don’t tell” policy of banning openly gay service members. “It was obviously very sensitive,” Gates said. “He managed to carry it off.”

At DHS, which opened its doors in March 2003, Johnson will confront three major challenges, according to administration officials: the evolving terrorism threat, including home-grown violent extremism; assisting the private sector in fending off cyberattacks; and immigration reform, which includes border security.

At the Pentagon, Johnson was involved in thorny policy issues of combating terrorism, including whether and when it was appropriate to kill terrorists with drone strikes and how to deal with suspects if they were captured. Sometimes he clashed with other administration attorneys, a fact he acknowledged in a speech at Yale University last year. “The public should be reassured, not alarmed,” he said, “to learn there is occasionally disagreement and debate among lawyers within the executive branch.”

He advised Gates on a cybersecurity agreement with then-Homeland Security Secretary Janet Napolitano in which DHS would task the National Security Agency — which falls under the Defense Department — to respond in the event of an attack on private-sector computer systems that run critical functions such as electricity generation, Gates said. That pact aimed in part to ease tensions between the two agencies over who had the lead in domestic cybersecurity.

Johnson, a former federal prosecutor and trial lawyer in private practice, is pragmatic, said Ben Rhodes, deputy national security adviser for strategic communications. “So on the one hand, he was an advocate for maintaining the option for military commissions, and on the other, he worked to reform them in a way that they could be an option.”

A graduate of Morehouse College, the nation’s only all-male historically black college, and Columbia University Law School, Johnson is the son of an architect and the grandson of a renowned sociologist. He owes his unusual given name — pronounced “Jay” — to his grandfather, who was visiting Liberia in 1930 when civil strife broke out and was saved from death by a tribal chief named Jeh.

On that “darkest” day in 2001 that killed almost 3,000 Americans, Johnson said at Obama’s side Friday, “I wandered the streets of New York . . . and wondered and asked, what can I do? Since then, I have tried to devote myself to answering that question.”

Greg Miller and Ed O’Keefe contributed to this report.