In March 2001, the wrestling world changed forever, when the WWF bought WCW, ending the Monday Night War.
With all this new talent at his disposal, WWE Chairman Vince McMahon created the Invasion storyline, which saw wrestlers from WCW, and ECW, attempt to destroy the WWF from the inside.
Named the Alliance, the group were led by Vince's own children, Shane and Stephanie, culminating in a Survivor Series elimination match, which saw the WWF finally vanquish their competition.
In the years since the Invasion though, fans have critiqued the storyline, feeling it hurt more Superstars than it helped, and often made little to no sense.
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Despite being one of the most maligned stories in wrestling, there is plenty of the WCW/ECW Invasion that remains a mystery.
Here are five things you didn't know about the WCW/ECW Invasion of WWE.
#5 Pay Per View success
The Invasion storyline may not be looked back at fondly by fans, but the numbers don't lie, and things were going well for the WWE at the time.
In July 2001, the company hosted an Invasion pay per view, which saw WWF Superstars take on Alliance stars, in a battle for brand supremacy.
And whilst the show had some interesting booking choices (namely 'Stone Cold' Steve Austin joining the Alliance in the main event), the pay per view certainly drew a crowd.
With an estimated 770,000 buyrate, Invasion was the highest grossing non-WrestleMania pay per view ever, a record the show still holds to this day. Winning six matches out of the 11 (including the match on Heat), the Alliance won the battle, but as we all know, lost the war.
#4 Rey Mysterio wasn't allowed to be involved
When the WWF bought out WCW, they got a lot of young, hungry stars along the way, and there were few more popular than Rey Mysterio.
A beloved cruiserweight in WCW, Mysterio would debut in the WWE in 2002, skipping the entire Invasion storyline, and has gone on to become one of WWE's most popular Superstars of all time.
In fact, the master of the 619 was so popular due to his incredible matches in WCW, that the WWF chose not to include him in the storyline, as they thought fans would be unable to boo him.
This was clearly the right call, as Mysterio has been constantly cheered for his underdog spirit by fans, and has gone on to become one of the company's most decorated champions.
#3 Fans thought it was boring
If you've seen anything about the Monday Night Wars, then you'll know the infamous moment of RAW, where Vince McMahon gave a promo about WCW, only for Shane McMahon to appear on the titantron, and reveal he had purchased WCW from under his father's nose.
A truly iconic moment, this was no-doubt a shocking moment for the fans live at RAW and Nitro, and was definitely an exciting moment for everyone involved.
At least, that's what WWE want you to believe.
Instead, the crowd at RAW were not fans of the iconic moment, with Vince's long victory speech getting boring chants from the crowd.
Whilst the magic of editing has since edited out these chants and now portrays it as an impassioned promo, fans who attended live know the truth.
#2 DDP wishes he wasn't involved
Rey Mysterio may not have been allowed to join the Invasion, but one man who certainly regrets taking part is Diamond Dallas Page.
A three-time WCW World Champion, DDP was one of the company's biggest stars, with fans appreciating his natural personality and charisma.
Debuting in the WWE, the company made Page the stalker of The Undertaker's wife Sara, which led to the Yoga instructor getting his butt handed to him by the Deadman for months.
Since the Invasion ended, Page has gone on record saying how he regrets taking part and felt it left a black mark on his career.
A former three-time World Champion, having Page as the stalker killed any possibility of being a world champion again, and he left the WWE the next year.
#1 Heyman's shoot
If there's one good thing that came from the Invasion, it took place on the final SmackDown before the Survivor Series.
With everything on the line, Paul Heyman, who had been the advocate for the Alliance called Vince McMahon to the ring.
In one of the greatest promos ever, Heyman lambasted the chairman, speaking about how Vince had betrayed everyone who had trusted him, to make money.
"You stole Bret Hart's dream... and bought yourself an airplane... you stole Shawn Michael's smile... and made yourself a billionaire."
The best part about the promo, which you can watch for yourself below, is that it was entirely a shoot, as Vince reportedly told Heyman to sell Survivor Series to the fans.
Whilst Heyman's side may have lost that Sunday, Paul E would be back months later, as the manager for a new rookie upstart: Brock Lesnar.