Watford were left to rue what might have been as Arsenal came away from Vicarage Road with a 3-1 win in the Premier League.
The Gunners found themselves three goals to the good at half-time through a contentious Santi Cazorla penalty, before Alexis Sanchez and Mesut Özil extended the lead just shy of the half-time break.
A much-improved second-half showing followed from the Hornets, with half-time substitute Roberto Pereyra scoring on his debut, curling the ball into the bottom corner after Capoue's effort had been blocked. But the damage was done before the break as Arsène Wenger's side claimed all three points.
Walter Mazzarri made two changes to his side which faced Chelsea in the Premier League a week earlier, with Younès Kaboul and Christian Kabasele coming in to replace the unavailable duo of Craig Cathcart and Miguel Britos.
Heurelho Gomes started in goal behind a back three of Kabasele, Sebastian Prödl and Kaboul. Nordin Amrabat operated on the right-flank with José Holebas on the left.
The midfield trio was made up of Valon Behrami, Adlène Guedioura and Étienne Capoue.
Captain Troy Deeney and Odion Ighalo led the line for the Hornets.
New signings Daryl Janmaat and Pereyra were on the bench for the Golden Boys, with Isaac Success also included as he returned from injury.
The game was an even affair up until the seventh minute when Arsenal were awarded a penalty-kick. Referee Kevin Friend adjudged Amrabat to have fouled Alexis Sanchez as both challenged for a ball into the box by Mesut Özil.
Santi Cazorla stepped up to take the kick and fired his effort low past a diving Gomes.
The match sprung into life midway through the half as both teams fashioned a number of chances. Firstly Gomes denied the Gunners as he saved Theo Walcott's strike to his left.
A quick counter-attack followed and the Hornets were almost level. Amrabat whipped a fierce ball into the box from the right which Koscielny got a boot to.
However his clearance sliced towards goal and Petr Čech pulled off an exceptional reaction save to keep his side ahead.
Holebas broke down the left before arrowing in a low cross which Deeney just flicked wide of the upright.
Amrabat was the next to go close as his cross from the right almost caught Čech out, but it ultimately went over the bar.
The Hornets were looking sharp and causing the Gunners problems. Kabasele met Capoue's corner but his glancing header just went wide of Čech's post.
However Watford had to remain alert at the back as the well-known pairing of Özil and Sanchez combined to great-effect. But the latters' shot towards the bottom corner was well saved by Gomes and Capoue blocked Cazorla's shot from distance on the rebound.
The duo were at it again but this time Gomes was required to save an effort from Özil at his near-post.
With their next effort at goal the Gunners were two goals up. Walcott was released down the right channel before playing a ball across the box which was directed towards goal by Sanchez.
Prödl got to the ball on the goal-line, and cleared it away, but referee Friend awarded the goal following a notification from the goal-line technology.
On the stroke of half-time Gomes produced an impressive save low to his right to deny Walcott from a tight-angle at the near-post.
But moments later Ozil finally had his goal. Sanchez curled a ball in from the left to his fellow front-man who darted in and headed his effort past Gomes who could do little.
Mazzarri opted to make a change at the beginning of the second-half with Pereyra coming on in place of Guedioura for his debut.
Shortly into the second-half Kabasele was replaced by another new-face in Daryl Janmaat, who was also making his Watford bow.
The Gunners were an impressive force offensively, Bellerin's ball into the box from the right just needed a touch but eventually evaded everyone and the Hornets held on.
Soon after Amrabat laid the ball off to Capoue on the edge of the box but his fierce shot was held by Čech.
But minutes later the Hornets had a goal back through Pereyra. Capoue fired another effort at goal which appeared to strike a hand inside the box.
The ball dropped to Pereyra who curled into the bottom left corner for a goal on his debut to give the Hornets hope.
The Hornets were still in the game however and almost had a second. Holebas hit a powerful effort from distance which Čech parried away, however he was quick to get up and deny Ighalo on the rebound.
Watford were camped in the Gunners' half and were inches away from scoring again as Ighalo's overhead-kick from a Holebas throw-in just flew over.
In his final change of the afternoon, Mazzarri brought on Success in place of Amrabat for his competitive debut.
The substitute had an instant impact, first releasing Holebas down the flank and then seeing a header saved by Čech following a Capoue corner.
Pereyra had made an impressive impact since coming on at half-time and he set-up Behrami nicely but the midfielder's volley flew wide of goal.
In the closing moments of the game, Gomes was forced to save from Sanchez who had dispossessed Kaboul just outside the box.
As the game neared an end, Success flicked a ball into the box but Deeney was just unable to keep his header down.
The biggest cheer of the day came as Arsenal's first yellow was shown to Jack Wilshere, the home crowd less than impressed with referee Kevin Friend's decisions on the day.
The full-time whistle followed and the Hornets can take pride in their second-half performance which gave Wenger's team much to think about.
HORNETS: Gomes (GK); Kaboul, Prödl, Kabasele (Janmaat, 52); Amrabat (Success, 77), Guedioura (Pereyra, 45), Capoue, Behrami, Holebas; Deeney (c), Ighalo.
Subs not used: Pantilimon (GK), Zúñiga, Watson, Sinclair.
Goal: Pereyra (57)
Yellows: Prödl (17), Deeney (60), Amrabat (77), Behrami (83), Holebas (89)
GUNNERS: Čech (GK); Bellerin, Holding, Koscielny (c), Monreal (Gibbs, 74); Xhaka, Cazorla; Walcott, Özil (Wilshere, 69), Oxlade-Chamberlain (Elneny, 69); Sanchez.
Subs not used: Ospina (GK), Giroud, Chambers, Coquelin.
Goals: Cazorla (9), Sanchez (40), Özil (45)
Yellow: Wilshere (90)
Referee: Kevin Friend
Attendance: 20, 545
Report by Ed Henderson