Two superb long-range shots in the first half at Emirates Stadium saw the Hammers take the lead through Jack Collison in the aftermath of a corner before Lukas Podolski responded in kind to level the scores before the break with a powerful drive from 25 yards.
Arsenal then burst into life early in the second half, with Giroud’s double sandwiching strikes from Santi Cazorla and Theo Walcott to send Arsene Wenger’s men cruising to an emphatic win which cements their spot in sixth place to give their UEFA Champions League hopes a welcome boost.
The first half-chance of the evening fell to West Ham in the eighth minute when Ricardo Vaz Te got his head to Guy Demel’s ball in but could not direct his shot towards goal, then Arsenal responded as Cazorla tried his luck with a long-range effort that was always rising.
The breakthrough came after 16 minutes and went the Hammers’ way despite a spell of Arsenal pressure. There was a hairy moment for Wojciech Szczesny as Matthew Taylor’s volleyed effort landed on the roof of his net and West Ham earned two corners in quick succession. The second was poorly dealt with by Giroud, allowing Collison to blast home a sizzling strike from the edge of the box.
It took Arsenal barely four minutes to respond as Podolski came back with a long-range effort of his own. The Germany international pounced on a deft pass from Jack Wilshere to hammer the ball into the net from 25 yards and level the scores at 1-1.
Podolski came close again after some brilliant build-up play from the hosts; Giroud slipping him through on goal and the winger skilfully avoiding James Tomkins’ challenge only to see his shot cleared off the line by Joey O’Brien.
At the other end, the Hammers were denied a huge chance to go ahead when Aaron Ramsey slid in with a crucial last-ditch block to see off Carlton Cole’s flick over Szczesny.
Arsenal hinted at what turned out to be a sensational second half early on after the restart when Walcott nearly forced an own goal from Tomkins, then moments later Giroud had stretched the Gunners’ lead with his first of the night, greeting a corner with a precise left-footed shot to leave Jussi Jaaskelainen with no chance.
That strike triggered an Arsenal onslaught that spanned 10 exhilarating minutes. Cazorla followed up Giroud’s goal as he deftly backheeled in from close range after a lovely one-two between Giroud and Podolski. The German was then involved again as he charged down the right to feed Walcott with a pinpoint cross and the winger made no mistake slotting home.
Giroud added his second after some more good work from Podolski, once again released by Wilshere in midfield, as he poked home the winger’s cross to leave West Ham floundering at 5-1 down.
Allardyce made changes as he attempted damage control, introducing Alou Diarra and Dan Potts in a double swap, while Podolski and Giroud made way for Andre Santos and Alex Oxlade-Chamberlain after dynamic displays which earned them warm farewells from the Emirates home crowd.
There was more bad news for the Hammers soon after those changes as Potts went to ground after an unfortunate collision and required lengthy treatment on the pitch before eventually being stretchered off to applause from all around the ground.
That exit left the atmosphere understandably subdued and while Arsenal could not recapture their goalscoring sparkle of the early minutes of the opening half, they continued to look comfortable as 12 minutes of stoppage time were added as a result of Potts’ injury.
Oxlade-Chamberlain was denied by a brave charge from Jaaskelainen as the minutes trickled down and Cazorla had two chances to stretch Arsenal’s lead further in the dying minutes but West Ham managed to hold off any more humiliation.