This is how international law changed after September 11 
9/11 Memorial New York
This is how international law changed after September 11 

4 coordinated terrorist attacks killed almost 3,000 people on 11 September 2001.

The GCSP’s Foundation Council President, French Ambassador Jean-David Levitte was President of the UN Security Council, which was responsible for maintaining international peace and security.

On September 11 he was in New York and witnessed a plane hitting the second tower.

“We saw a plane crashing into the second tower. We thought it was a fire. But suddenly, we understood that it was not the fire, it was war.” 
As many as 100,000 people could have been inside the towers (Source: BBC)

“I was convinced that there were 50,000 people dead. Because we didn't know that they were evacuated in a very orderly process, thanks to the fire fighters.” 
Ambassador Levitte had to act immediately.

“We were in charge of the Security Council... And I said, we have to take an initiative because this is a turning point in world history, we have to react.”
They prepared a resolution to change the rules within the UN Charter 

“We said any attack against any country, by a group of international terrorists should be considered as an act of war. And under the control of the Security Council of the UN, the country being attacked has the right of self defence against the states, which offered hospitality, training, financing to the international terrorist group.”
The changes were unanimously accepted by the UN Security Council

“On September 12, at 10 o'clock, we were at the Security Council room to discuss the text and after one hour the 15 members were in agreement. So we had a very solemn meeting with Kofi Annan, the Secretary General to adopt the text, and I proposed that all the members of the Security Council would approve the text not by raising their hands, but by standing first to pay homage to the victims of 9/11.”