But as much as the hosts impressed in a busy performance – full of pace, movement and intent – West Brom were abysmal, showing just why they had lost 10 in a row away from home before last week’s last-gasp draw against Derby County.
Chaplin could have made it 1-0 inside two minutes with a flicked effort that went wide at the near post and he also went close with a drilled strike across goal before calmly slotting Pompey ahead from Alli’s measured pass.
Alli’s equally cool one-on-one finish rewarded continued attacking purpose and pressure, while Terry Devlin and Adrian Segecic also had chances to add to the advantage.
Adams did increase the advantage within four minutes of the restart, firing home from Chaplin’s cushioned volley to as good as seal victory and further demoralise a West Brom side who have now already lost 12 matches away from home this term.
Mikey Johnston did manage an attempt on target for the Baggies – their only one of the game – when his curling long-range strike was well tipped over by Pompey keeper Nicolas Schmid.
But Portsmouth remained in full control and could have easily added to their lead, with the only blemish an ankle injury suffered by Colby Bishop in the second half.