"largest worldwide non-violent protests in history"? I remember living through that time and don't remember that. Do you have a source? I myself was opposed the second Iraq war because Saddam had agreed to let in any inspectors the west wanted but we went "too late, we're coming in anyway" and I knew it was a scam invasion.
We were also just a couple of years into Afghanistan and it made no sense to be starting a second war on a second front when there was no immanent danger. Again, it made to sense.
On February 15, 2003, anti-war rallies took place across the globe – the largest occurring in Rome, Italy, where a crowd of 3 million gathered to protest against the USA’s threat to invade Iraq. Police figures report that millions more demonstrated in nearly 600 cities worldwide: on the same day, 1.3 million rallied in Barcelona, Spain, 1 million participated in a peace march through the streets of London, UK, and 500,00 people in Melbourne and Sydney, Australia, joined the biggest marches since the Vietnam War peace protests.
The French political scientist Dominique Reynié has estimated that, between 3 January and 12 April 2003, some 36 million people took part in nearly 3,000 protests around the world against the Iraq war.
(It's worth noting here that I have been unable to find Dominique Reynié's paper that estimates this. I have searched and searched for a PDF with no luck. Lots of references to this work, but can't seem to find the actual document.)
Between six and 10 million people are thought to have marched in up to 60 countries over the weekend - the largest demonstrations of their kind since the Vietnam War.
A key aspect of what made it so big was because it was happening worldwide, simultaneously, in multiple cities all over the world.
I was a bit skeptical as well, but there's at least one seemingly reputable academic researcher who says as much: https://en.wikipedia.org/wiki/15_February_2003_anti-war_protests (first citation).
So even if it wasn't, one could easily be forgiven for the mistake.