WASHINGTON — In the five years since terrorists flew hijacked airliners into the World Trade Center and the Pentagon, Americans have accepted inconvenience, sacrificed personal liberties and paid billions of dollars for a security clampdown that touches virtually every aspect of their lives.

And we’re still not safe.

A close examination of the federal government’s homeland security effort shows that there have been major accomplishments since the attacks on Sept. 11, 2001. But it also reveals how vulnerable the nation remains to catastrophe.

Federal officials and security experts directly involved in the cat-and-mouse game with terrorists have realized that the nation faces more threats than the government can ever combat. The result is a deadly guessing game on a global scale, with security officials often one step behind the terrorists.

“We can’t cover every conceivable target against every conceivable attack at every waking moment,” said Bruce Hoffman, a Georgetown University professor in security studies. “You strengthen one set of targets, they’ll shift to another.”

The threat of terrorism against the United States remains chillingly lethal five years after 9/11, and officials predict another massive attack is not a matter of if — but when.

The nation has spent more than $280 billion on the domestic side of the war on terrorism over the past five years to hire thousands more FBI and Border Patrol agents and buy high-tech devices to secure the nation’s planes, trains, ports, nuclear reactors and other potential targets. U.S. military operations in Iraq and Afghanistan have cost $400-plus billion more.

All this, that the nation hasn’t been hit since Sept. 11, 2001, may say as much about terrorists’ patience as it does about steps taken to stop them.

“I know of nobody in the intelligence field who doesn’t believe there will be another attack,” said Thomas Kean, former New Jersey governor and Republican chair of the 9/11 Commission that investigated the government’s security missteps leading up to the 2001 hijackings.

“There’s going to be another attack,” Kean said. “They just can’t tell you when.”

In a new age of rapid and widespread ID checks, locked and bulletproof cockpit doors in airliners, armed pilots, tracking foreigners’ visas and monitoring Muslim and Arab communities, few expect a precise repeat of the plot that used airline hijackings to bring down big buildings.

The unsettling reality of terrorism, however, is that it is always in search of new ways to accomplish mass death and destruction.

Authorities have disrupted a number of high-profile plots, including last month’s bombing scare on as many as 10 Britain-to-U.S. flights. The CIA has helped ensnare some 5,000 terror suspects around the world. And the government has imposed hundreds of security measures on foreign visitors and U.S. residents alike, from making travelers take off their shoes at airport checkpoints to eavesdropping on phone and e-mail conversations.

But glaring gaps in the security net remain.

Undercover inspectors testing the nation’s security system have repeatedly sneaked weapons through airport checkpoints, entered the country with fake identification and foiled detectors that catch the trace amounts of radiation in kitty litter and bananas, but not always nuclear materials. Air testers to sniff out biological agents are becoming obsolete. And not all port or airline cargo is rigorously inspected.

And, as Hurricane Katrina showed last year, disaster response systems at all levels of government are woefully unprepared for a catastrophe.

“No matter what you do, it’s not enough,” said Rep. Carolyn Maloney, D-N.Y., co-chair of a congressional 9/11 caucus. “But the systems we’ve worked hard on to put in place are not working.”