In theory, after WWE bought the assets of WCW, Booker T defending his WCW Title against Buff Bagwell was supposed to be the moment that really got the "interpromotional" invasion angle going.
There were a few months of teases and WCW wrestlers interfering in matches, but this was different. It was a WCW match with WCW announcers, a WCW referee and WCW-style lighting to get fans ready for the angle where Raw became the WCW brand show.
Instead, the match was bad, the crowd turned on it, and the plan for a separate full-time WCW brand was immediately dropped.
The new plan was unveiled a week later in Atlanta, which had been the home of WCW. During a Kane and Chris Jericho (WWE) vs. Lance Storm and Mike Awesome (WCW), the recently unemployed (thanks to ECW folding and going bankrupt) Rob Van Dam and Tommy Dreamer emerged from the crowd in a chaotic scene.
It wasn't long before a group of WWE stars (the Dudley Boyz, Rhyno, Tazz, and Justin Credible) emerged from the locker room. They stared down the two groups of invaders and...wait a second...that's a weird group of guys to make the save...
They turned towards Kane and Jericho, who were soon attacked by everyone else in the ring. Heyman left the announcers' desk to announce that ECW was back.
it was a very chaotic episode of Raw that seemingly burned through months of storylines: Shane McMahon turned heel to start the show, ECW was reborn in the middle, and the show closed with both ECW and WCW joining forces as the Alliance and the announcement that Stephanie McMahon bought ECW. On top of that, the invasion quickly went downhill.
Still, the ECW reformation happily exists in a vacuum, a great segment that, thanks to brilliant execution, can somehow stand on its own in our memories in spite of being a tipping point in what should have been the greatest feud of all time fizzling out.