NATO’s secretary general Jens Stoltenberg has insisted that the defense alliance’s commitment to support each other militarily is just as crucial for U.S. security as it is for that of Europe and Canada.

“NATO is important for Europe but it’s also important for the United States,” Stoltenberg told CNBC Wednesday in a bid to preserve the group’s commitment to this central principle after President Donald Trump failed to endorse it at a meeting with NATO members last month.

The agreement, known as Article 5, has only been triggered once by the group, when European and Canadian members joined the U.S. in its war in Afghanistan following the September 11, 2001, terror attacks.

“Hundreds of thousands of Europeans and Canadians have fought alongside U.S. soldiers in Afghanistan to fight terrorism and prevent terrorists again using Afghanistan as a base for attacks against the U.S. and other countries,” Stoltenberg said ahead of the latest meeting of NATO defense ministers in Brussels on Thursday.

The President received criticism last month when he failed to mention this clause, despite it being included in a prepared speech. It is not clear, however, whether he personally omitted the reference or it was removed by White House advisers.

However, Stoltenberg said that the group remained committed to continuing its combined efforts in Afghanistan.

Tomorrow the group is due to discuss possible plans to send a further 3,000 troops to join the 13,000 already deployed in Afghanistan.