Southampton supporters welcomed their team in their thousands, gathering outside the ground with red and white flares, lining the streets more than two hours before kick-off.
Pyrotechnics and flamethrowers added to a thrilling pre-match atmosphere, but it took until Smallbone's goal for the game to heat up.
Saints first came the closest to opening the scoring in the first half, Armstrong getting in behind the West Brom defence before cutting the ball back to David Brooks who fired wide from close range.
Russell Martin’s home side started the second half impressively and the atmosphere erupted when they took the lead through the boot of Smallbone, who netted his seventh goal of the season.
Saints supporters felt they should have had a penalty after Brooks was scissor tackled by Kipre in the box but, to their dismay, nothing was given.
Armstrong settled the nerves later on, drilling a low shot into the back of the net before celebrating in the far corner of the Northam stand.
The hosts were eventually given a penalty in the closing stages when Manning steamed into the West Brom box before being taken down, with Armstrong making no mistake from the spot.
Kipre's header through the outstretched gloves of Alex McCarthy was too little too late for West Brom.
