As Spain emerges from three days of mourning and investigates Thursday's commuter train bombings, the United States not only mourns with it, but also must help its ally in the war on terror deal with such a horrific tragedy.

Many Spaniards will forever equate Bloody Thursday with the 9-11 attacks on the World Trade Center, and rightly so. Ten bombs blasted through four trains in orchestrated attacks during rush hour. At least 200 people were killed and 1,400 were injured. Many of them are in the hospital fighting for their lives.

It was the worst act of terrorism in Spain's modern history and the deadliest in Europe since December 1988, when a bomb exploded in a Pan Am jet over Lockerbie, Scotland, killing 270. Libya recently paid millions to the families of victims in taking responsibility for that crime.

The security lessons we learned from Sept. 11 will be of vital importance to Spain now. The country's lax security on its commuter trains made it an easy target for terrorists. The bombs were reportedly in backpacks.

America and Spain, as well as other European countries, need to treat this tragedy as a wake-up call, continue to share intelligence information and redouble their efforts to crack down on terrorist organizations. No country can stand alone in the face of global terrorism, no matter who the perpetrators are, but especially if the bombers turn out to be linked to Osama bin Laden's al-Qaida.

Terrorism-related carnage on innocent civilians, such as that seen Thursday in Spain, on Sept. 11, 2001, in the United States and in October 2002 in Bali, is meant to cause mass fear and hysteria. But strength and decisive action ought to be the response.

At first, Spanish officials believed the perpetrators were part of the separatist Basque group ETA, which has killed hundreds there over the years. It occurred just before Spain was to elect a new prime minister. However, an organization linked to an al-Qaida general claimed responsibility through a letter sent to a London newspaper. There also is speculation that the two groups could be collaborating.

Spain has been one of the United States' strongest European supporters in the Iraq war and in the war on terror following Sept. 11. During his eight-year term, Spanish Prime Minister Jose Maria Aznar has made stopping terrorists a priority.

The escalation of terrorism since 9-11 underscores need for any world leader, particularly one whose country has been targeted, to be on guard. It's part of the post-9-11 world in which we live that includes intrusive searches, terror alerts and added security around mass transit sites.

Spain will recover, just as the United States has, but it must now always be mindful of the brutal impact of terror and - with help - be prepared to resist it.