With more than 50 killed and 700-plus wounded, last week's horrific terrorist attacks in London serve as a stark reminder that we have two enemies in the War on Terror: al Qaeda - and complacency. I'm not suggesting that the plucky Brits were complacent. They've been living with - and fighting - terrorism (especially the IRA) much longer than we Yanks have.

In fact, from 9/11 until last week, the Brits had been remarkably successful - breaking up six al Qaeda terrorist plots before they could be put into action.

But Thursday's attacks by the previously unknown "al Qaeda in Europe" demonstrate that even a nation well prepared for - and steeled by - terrorism can be bloodied badly by it. We'd better take heed.

Since 9/11, al Qaeda has struck successfully in places such as Bali, Indonesia (2002: 202 killed); Madrid (2004: 191 killed); Hilla, Iraq (2005: 125 killed), and now London - nothing here in America.

But make no mistake about it - the real brass ring for al Qaeda remains nailing an American target like New York City - again.

A July 7 Department of Homeland Security (DHS) memo warned that al Qaeda has wanted to duplicate a Madrid-style passenger train bombing (10 backpack bombs on rush-hour trains) in the United Kingdom or the United States. After last week's attack, that leaves us.

Think about it: More than 7 million tristate commuters hop on buses, trains and subways in the metro area each day. That's three times London's daily passenger load - talk about a target-rich environment.

But likely al Qaeda terrorist scenarios aren't limited to bombing or derailing trains, according to DHS's Joint Information Bulletin. Despite the up-tick in airport security, al Qaeda remains keen on attacking aviation.

Their options extend beyond a 9/11-style attack in which operatives seize planes and turn them into veritable cruise missiles. Alternatives include planting operatives in flight-operations areas, where they can load explosives disguised as "seemingly innocuous items" onto aircraft.

Buildings are also on al Qaeda's target list, warns DHS. One scenario involves "renting several rooms in a targeted building," filling them with natural gas, and exploding them BY using a timer placed on the ceiling.

DHS is also concerned about tanker trucks carrying flammable liquids. Imagine the effect of a thermal fireball caused by a tanker explosion in the Midtown or Holland Tunnel.

What the homeland-security report doesn't mention is a possible attack here with a non-conventional weapon, such as a nuclear or radiological "dirty" bomb.

If detonated on Wall Street, even the less powerful dirty bomb (which disperses nuclear material using a conventional explosion) could leave blocks of the financial district completely uninhabitable until the radioactive buildings are carted away.

The memo's silence on this possibility is understandable - happily, chemical, biological, radiological and nuclear weapons are still tough to acquire.

It's much easier to lay one's hands on (and work with) conventional explosives. But, as demonstrated so painfully in London - and Iraq - these old-fashioned weapons can be used with deadly efficiency.

The good news is that the July 7 memo concludes - and DHS reconfirmed over the weekend - that there is "no specific credible information to indicate that an attack on the United States is imminent." Nor does DHS believe that al Qaeda terrorists are now in the United States to conduct a strike.

The bad news is that ongoing intelligence indicates that al Qaeda remains intent on attacking America directly. And the fact is, neither we nor the British had any warning of Thursday's attacks.

Was this a 9/11-like intelligence failure on the part of Britain's highly-competent security services, or is al Qaeda getting better at its terrorist tradecraft?

We may not know that answer for quite some time, as the British authorities investigate the tragic event and hunt down the killers. Unfortunately, it might be the latter. (Also in the mix: The chance that, despite the name, this group is homegrown, with no real connection to Osama's boys.)

Despite robust counterterrorism efforts across the globe over the last four years, al Qaeda is still finding ways to lash out and kill innocent civilians.

We can only conclude that they're adapting to new anti-terrorism initiatives, perhaps even becoming more skilled at their terrorist black arts, especially in terms of operations and communications security.

For example, the terrorists aren't using intercept-vulnerable phones to communicate, but instead rely on couriers and the Internet to coordinate attacks - once again highlighting the importance of human intelligence.

We're facing a tougher enemy; we can't let our guard down - or stop pressing toward victory in the War on Terror - even for a second.

Peter Brookes is Heritage Foundation senior fellow.