Editor's note: Fox News contributor and former U.S. Ambassador to the United Nations John Bolton was Under Secretary of State for Arms Control and International Security when the 9/11 terror attacks occurred.

I arrived at the State Department early on September 11, my bags packed to fly to London that afternoon. I was to meet there with Georgi Mamedov, my Russian counterpart, continuing negotiations to extract the United States from the outdated 1972 Anti-Ballistic Missile Treaty. Secretary Colin Powell had a September 19 meeting with Russian Foreign Minister Igor Ivanov, and there were summits between Presidents Bush and Putin in Shanghai and Crawford, Texas, in October and November to prepare for.

Typically, State has morning and periodic meetings so the bureaucracy can get its act together, both for the day ahead and the longer term. We were just finishing one of those meetings -- evaluating careerists for vacant ambassadorial positions -- when we received word that two planes had crashed into the World Trade Center’s Twin Towers.

None of us had the slightest idea what was up. I hurried back to my office in the southwest corner of the Department’s seventh floor, the floor that housed the Secretary of State and other high-ranking personnel, to learn more about the bizarre-sounding developments in New York.

At some point a few minutes later -- I have no recollection of the exact time -- my secretary raced into my office to say that the Pentagon was on fire. I went to a window and saw a great cloud of smoke and fire rising just across the Potomac River from the iconic building’s western side. Another staffer then ran in to exclaim that he had just watched a large aircraft crash into the Pentagon right next to its helipad.

We were under attack. We had no clue what was coming next.

I went immediately to the State Department Operations Center, in the middle of the seventh floor, where several top officials were gathering in a secure videoconferencing facility so we could communicate with other government agencies.

With President Bush in Florida, we knew that the Vice President and other key White House personnel had been moved to a bunker deep under the White House. NSC staffers were in the Situation Room in the West Wing basement, trying to coordinate information from around the country to determine if there were further potential attacks or hijackings under way.

Everything seemed to be in chaos, then and for much of the rest of the day. For hours, there were worries about a flight hijacked from Dulles, which we realized only later was not still “missing” and potentially dangerous, but which was the plane that had struck the Pentagon. It took until early afternoon to conclude that four aircraft had been hijacked domestically, although it was still far from certain what was happening on board international flights heading toward America.

Although the nation as a whole was understandably focused on the attacks and the obvious and immediate human tragedies involved, the White House, the State and Defense Departments, and others began automatically to concentrate on what would come next. Some of State’s activity on September 11 and thereafter was vital but quintessentially bureaucratic: letting the rest of the world know the U.S. government was still functioning. We activated the Cold War “hot line” to Moscow, for example, to confirm earlier high-level messages that enhanced U.S. military readiness was in response to the attacks, not in any way related to Russia.

That day, Powell was in Peru for long-scheduled meetings, the remainder of which he immediately cancelled, returning to Washington in the early evening. On September 12, after a large 7:00 A.M. meeting in his office, he asked me to stay behind. Powell wanted me to do whatever it took to reschedule the lost meeting with Mamedov, in advance of Russian Foreign Minister Ivanov’s impending arrival in Washington. Keeping that Powell-Ivanov meeting was important substantively, but Powell also wanted to send Russia and other countries yet another signal, however small, that America was fully open for business.

With commercial air travel still chaotic, I finagled my own “milair” plane to Moscow, an Air Force version of the Gulfstream V, departing Andrews Air Force Base on September 15 with a small delegation. Our course took us just east of Manhattan, and we stared at the huge cloud of dust and smoke still billowing from where the World Trade Center had stood. We watched in silence. There was simply nothing to say.

U.S. intelligence had missed Al Qaeda’s preparations for the 9/11 attacks, and there was no telling what else we were missing. And, as devastating as 9/11 was, the consequences of a terrorist or rogue-state attack with nuclear, biological or chemical weapons, possibly delivered via ballistic missile, would be far worse. We formally withdrew from the ABM Treaty on December 13.

After 9/11, proliferation and terrorism were not “problems” to be “managed.” They were mortal threats to be stopped. They still are.

Former U.N. Ambassador John Bolton, is a Fox News contributor and a senior fellow at the American Enterprise Institute, is the author of "Surrender Is Not an Option: Defending America at the United Nations" (Simon & Schuster, 2007).