When I joined the Canadian Forces (CF) in 2000, our main focus was still fighting against fictional countries from the Warsaw Pact. We used to breach defensive positions, establish them with the help of the engineers, conduct combat team attacks on prepared positions, etc.

But when the two towers went down on 9/11 our focus drastically changed. As we drove NATO into adopting the article 5 to support our American brothers and sisters, the CF knew that their current doctrines were outdated and were not ready to go fight an unconventional warfare (UW) like they would do for the next 13 years. It was the first time in the Alliance’s history that Article 5 has been invoked (source) as well.

This is the unconventional warfare definition drawn from the FM 3-05.201, (S/NF) Special Forces Unconventional Warfare (U):

Operations conducted by, with, or through irregular forces in support of a resistance movement, an insurgency, or conventional military operations.