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In the wake of the Moscow bombings, some global security experts say this kind of violence cannot be permanently 1 stopped. But governments can do more to prevent mass casualties. Our correspondent takes a look at the additional measures governments can take and what terrorists hope to achieve through violence.

It's a commuter's worst nightmare, a normal day turned upside down by an explosion.

But some global security experts say the Moscow bombings represent a kind of terrorism that is here to stay.

"This kind of violence cannot be permanently stopped, and that we may be living with this for the rest of our lifetimes," noted 2 Michael O'Hanlon of the Brookings Institution.

In the wake of the bombings, cities in the United States and Europe boosted security in and around their mass transit 3 systems. 

But the heightened security will only last a short time.  O'Hanlon says there should be more permanent changes in mass transit security.

"I do believe we probably should step up the relatively 4 unobtrusive and relatively easy, although sometimes somewhat expensive, means of looking for explosives perhaps many more K-9 bomb teams at these kinds of places and we should put a lot of resources in intelligence work," he added.

While terrorism is a constant threat, local and federal emergency responders in the U.S. regularly conduct drills to prepare for disaster.

The goal of these exercises is to practice, so when a real emergency hits these crews will be prepared.  Lindsay Godwin of the Federal Bureau of Investigation 5.

From the tragedy in Moscow's subway, to the 2004 Madrid train bombings and the explosions in London's public transit system the following year, experts say no country is immune to terrorism.

"Today, the goal of modern apocalyptic 6 terrorism is often to do as much disruption as possible and kill as many as possible to really shake up the way in which societies operate and governments make decisions and make the pain so high that decisions are reassessed," explained O'Hanlon.

O'Hanlon says even though the aim of modern day terrorists is to force a change in policy, so far, the tactics do not work.

"What I think some of these terrorist group may have underestimated is that we in the west or in Russia, in this case, are perhaps not quite as soft as they might have first believed,"  he added.

O'Hanlon says since the September 11 attacks, Western societies have not tolerated terrorism.

And cities across the United States will continue to prepare for the worst, in exercises like this one, so if another attack occurs, they will be ready for it.