Wig
"Article 5 Collective defence means that an attack against one Ally is considered as an attack against all Allies." 

"attack" is a key word. For example if a NATO country attacks a non NATO country then if the non NATO responds militarily it certainly is not an attack. It is retaliation. However NATO went to the support of the US even though it had been involved attacking Arabs, Palestine for example. Under those rules a NATO country can get help from NATO by attacking another country. 

The US had been involved in supplying arms and money for takeover of land in Palestine. When the Arabs responded, the US claimed Article 5 and NATO countries went along with it. Clearly it was mutual offense that followed. 

Our PM at the time, Chretien Should have blown the whistle. Saved what 154 Canadian troops and multitude of PTSD cases, many to come.