Who is referred to as 'The Swiss' in Arms and the Man by George Bernard Shaw?

In general therms, "the Swiss" are citizens of Switzerland. Historically, and even through the present day, Switzerland tended to remain neutral in the great wars tearing apart Europe, and thus a meeting ground for diplomats. 

Although Switzerland itself was neutral in the Serbo-Bulgarian war, the Swiss preserved their independence by requiring all males to have military training and thus were notable as mercenaries in this period. Many of the Swiss cantons maintained militias that hired...

In general therms, "the Swiss" are citizens of Switzerland. Historically, and even through the present day, Switzerland tended to remain neutral in the great wars tearing apart Europe, and thus a meeting ground for diplomats. 

Although Switzerland itself was neutral in the Serbo-Bulgarian war, the Swiss preserved their independence by requiring all males to have military training and thus were notable as mercenaries in this period. Many of the Swiss cantons maintained militias that hired out as mercenary groups, known for their skills as professional soldiers.

Captain Bluntschli is the only Swiss character in the play. He is one of a group of Swiss mercenaries who fought for the losing Serbian side in the war. At the start of the play, he climbs through the window of a Bulgarian house. In the context of the play, "the Swiss" refer collectively to the Swiss mercenaries.