The Central Criminal Court has been told that two Dublin youths went looking for a fight and are both liable for attacking a German student and murdering another.
Wesley Kelly, 20, from St Anthony's Road in Rialto and a 17-year-old, who cannot be named for legal reasons, have pleaded not guilty to the charges.
Thomas Heinrich and Robert Rinker were stabbed in Rialto in Dublin on 1 December 2012.
Mr Rinker survived, but 22-year-old Mr Heinrich died.
The two German students met and became friendly while on a one-year Erasmus study programme at Griffith College.
They had been on a night out and went back to Mr Heinrich's apartment on St Anthony's Road at around 4.30am on the morning of 1 December 2012.
The Central Criminal Court was told that the two students went out on the first floor balcony for a cigarette but words were exchanged between them and two youths passing on the street below. A can of beer was also thrown.
The prosecution counsel told the jury that the 17-year-old on trial took umbrage at something Mr Rinker said, went off and got Mr Kelly, returned and got into a row with the German students who had come down to the street.
In her opening statement, Senior Counsel Tara Burns also said that when the German students saw a knife they went back upstairs and came down again with blades.
Ms Burns said Mr Rinker said he was stabbed at the front of the apartment and ran back upstairs.
Mr Heinrich followed a short time later bleeding heavily. He was taken to hospital but later died there.
The prosecution says it is not possible to say who did what at what time but that Mr Kelly and the 17-year-old acted in a joint enterprise, went looking for a fight and are jointly liable for the assault and the murder.
Both youths deny the charges and the case is expected to last two weeks.