might be me, but seems like this is the clossest the world has been to a world war (since the end of world war 2)
Cuban Missle Crisis in October 1962. American and Russian naval forces were armed and if not for the Russian ships turning back America and Russia would have been in an active shooting war. That would be a world war, no way those two countries get into a shooting war and it doesn't pull in many other nations. Forces in 1962:
NATO: America, Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal, the United Kingdom, Greece, Turkey and West Germany
USSR: Today that consists of Russia, Armenia, Azerbaijan, Belarus, Estonia, Georga, Kazakhastan, Kyrgyzstan, Latvia, Lithuania, Moldova, Tajikistan, Turkmenistan, Ukraine and Uzbekistan.
That is 30 countries by todays standards. Plus I cannot see olaces like Australia and Japan (heavy american control) sitting it out.
ISIS does not strike me as a world war level threat. not when countries like Israel, America, Iran and Russia are all on the same side.