West Bromwich Albion scored four times in the final 20 minutes of the first half
West Bromwich Albion made boss Tony Mowbray's Hawthorns return a memorable one as they scored four first-half goals in a convincing demolition of Portsmouth.
Taking charge of a home game for the first time since returning for a second spell in charge of Albion, Mowbray watched Alex Mowatt open the scoring with a low strike.
Grady Diangana doubled the Baggies' lead after a parry from Portsmouth keeper Nicholas Schmid fell into his path.
Captain Jed Wallace made it 3-0 before Diangana pounced for four before half-time and John Swift scored a fifth in the second half to add to Portsmouth's miseries.
John Mousinho's Blues notched a late consolation goal through debutant and new signing Thomas Waddingham with the last kick of the game.
The result lifted Mowbray's Albion up to fifth in the table after Middlesbrough and Blackburn Rovers both lost.
Portsmouth drop to just two points clear of the Championship's bottom three in 21st position and have won just the one league game on the road this season.
A rotated Portsmouth side made five changes from their home win against Stoke City in midweek but went closest early on as striker Colby Bishop flicked the ball into the path of Andre Dozzell whose effort on the half-volley was well saved.
Moments later, Swift attempted to curl a shot into the far corner but Schmid made the save and Diangana then launched an effort that flew just wide.
The hosts then took the lead through Mowatt, who had scored twice in the return league fixture at Fratton Park.
He received the ball outside the box and struck a thunderous low drive into the bottom right-hand corner that Schmid managed to get a hand to but could not keep out.
The hosts soon doubled their lead as Mikey Johnston beat two defenders before launching a shot at goal. Schmid saved but his parry only went as far as the waiting Diangana.
Diangana then turned provider, laying the ball off to Wallace, who carried on his run into the penalty area before slotting the ball home and wheeling off in celebration.
The damage got even worse for Portsmouth just before the break as Diangana was played in by Mason Holgate and made no mistake.
Swift added a fifth just before the hour mark as the midfielder received the ball in space on the edge of the box and sent it into the top corner via a deflection.
With just over 10 minutes remaining, Albion almost had a sixth as Tom Fellows was played through one-on-one with just Schmid to beat but was unable to find a finish.
Portsmouth did eventually score with the final kick of the game as Waddingham came on for his debut and fired home at the second attempt after a low cross from the right.
No 'ego about tactics' - Mowbray
West Bromwich Albion boss Tony Mowbray speaking to BBC Radio WM:
"The first 20 minutes we were under the cosh and were caught out a bit, they pressed really high and really well.
"When we did get the first goal it unleashed us and probably put some demons in their minds - our two wide players were so good today.
"I have to inspire my team and I don't have an ego about my tactics - I have an understanding that if I give these talented players a licence and space to express themselves then we can win a lot of football matches."
Away defeats like this happening 'far too often' - Mousinho
Portsmouth boss John Mousinho speaking to BBC Radio Solent:
"It's really frustrating and disappointing. We were beat by the better side, there's no doubt about that, West Brom are a really good team.
"I was really pleased with our start and the opening 20-25 minutes, we had the better chances, created more and had a few shots.
"Then we conceded and then we just went under, we got punished. It seems to be something that happens far too often at the moment.
"When we go a goal down away from home confidence drops and we don't have the ability at the moment to dig in and find something extra."
