Gabriel Jesus scored 90 seconds into his first game for the Gunners, and his double helped the team to a comeback victory over FC Nurnberg.
After coming on at half-time in Germany, the new signing from Manchester City made an immediate impact with a goal.
Mohammed Elneny hit a wonder-strike to equalise for Mikel Arteta's side, before two Nurnburg own goals gave the Gunners a 4-2 advantage.
Jesus finished Gabriel Martinelli's cross at the front post in the 15th minute of the second half to make it a five goal game for the visitors.
The warm-up game fell into disaster after Arteta named a strong starting line-up.
In the space of five first-half minutes, Johannes and Duah scored against Matt Turner, who was making his first appearance for the club.
The international players are expected to return next week before the tour of the US.
West Ham started their pre-season campaign with a 2-0 win over Servette.
Kurt Zouma, Issa Diop, Michail Antonio, and Said Benrahma were included in the line-up for the first friendly of the summer.
The Hammers took the lead when Lanzini's free-kick caused havoc at the back, leaving youngster Ali Ajese to tap home into an empty net.
There were six changes at the break and one of them was Divin Mabumba, who headed home the rebound after Benrahma's shot was saved.
The Hammers won despite Servette causing some problems and hitting the woodwork two times.
Newly-promotedNottingham Forest lost their first pre-season game since becoming a premier league club, as they were beaten by a trio of goals.
The club came back from behind to win.
Guilian Biancone signed a three-year contract at the City Ground last week after joining from Troyes, which was a positive for Steve Cooper, who scored his first goal for the club.
The transfer window will close at 11pm on September 1st.
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