Arsene Wenger’s side followed up the 3-0 victory over Manchester United with the same scoreline at Vicarage Road, but it did not reflect Watford’s performance.
The visitors started brighter, but Quique Sánchez Flores’ men grew into the game and they were unlucky not to be leading at the break. They came out buoyed in the second half, but once Arsenal broke the deadlock, the flow of the game changed.
The Gunners, who netted in quick succession in the win over Louis van Gaal’s men before the international break, took the lead through Alexis Sanchez, who has been with the Chile squad.
Substitute Olivier Giroud soon doubled their tally, before Aaron Ramsey added another for Arsenal’s third goal in just 12 minutes.
Arsenal nearly took the lead after a promising start. Ramsey crossed from deep, but Theo Walcott’s header was tame, allowing Heurelho Gomes to get down well to make the save.
Gomes was on hand again minutes later, this time denying Sanchez, who arrowed a dipping shot from 20 yards.
Watford attacked at pace, with Petr Cech making a good stop at the other end to prevent the lively Troy Deeney from getting onto the scoresheet. The Hornets captain had started well, displaying a tireless work-rate by closing down Arsenal players.
The striker had another chance to open the scoring as the ball fell to him in the box after 17 minutes, following good wing play by Ikechi Anya, and Deeney hesitated before Laurent Koscielny closed him down.
As Watford grew into the game, another fast attack saw their front two through on goal, with only Koscielny to beat en route to goal. Deeney tried to play it across the box and the French defender made a vital interception to deny Ighalo.
Ighalo could have netted soon after, as he dragged a shot wide when he was virtually through on goal. He tried to make amends by sliding a pass across the box, but Hector Bellerin cleared.
Arsenal pressed for an early second-half goal, with Mesut Ozil bursting down the left of the box, squaring across the six-yard box but Sebastian Prödl was well placed.
Vicarage Road started to sense the game was there for the taking and they got behind the Hornets as the tempo of the game increased.
Arsenal broke the deadlock shortly after the hour mark, when Ozil looked to be through before he was blocked and the ball fell fortuitously to Chilean Sanchez, who coolly dinked the ball over Gomes.
The Gunners soon doubled their lead, Giroud converting moments after replacing Walcott, after Ozil crossed, with the Frenchman coolly volleying into the roof of the net with his right foot.
It took just six minutes for Arsenal to net a third, when Bellerin made a dangerous run inside from the right, laying off the Welshman in the box, who coolly slotted past Gomes, albeit with a slight deflection.
Stopper Gomes was on hand to stop a dangerous header by Giroud late on, but Arsenal’s three strikes took the sting out of the match, as they now turn their attention to Tuesday’s Champions League clash with Bayern Munich.