Sissi, I've responded to this "singular/plural verb with collective nouns" thing so often that it's becoming quite tiresome...but here goes again...
Weekly in The Times, a �Feedback' column responds to readers' complaints. On March 10th 2007, one had written (abbreviated),,,
"I am writing to highlight the increasing incorrect use of collective nouns with plural verbs...'the team are doing this' or �England are doing that'..."
The editor responded...
"Actually it isn't incorrect. To quote Gowers: �In using collective words or nouns of multitude (department, government, committee and the like...there is no rule; either a singular or a plural verb may be used.'"
The Gowers referred to was Sir Ernest Gowers, the author of The Complete Plain Words (TCPW) first published in 1954. His words are echoed by those of the editor of Fowler's Modern English Usage (FMEU), who expresses the selfsame thought as, "In British English, collective nouns may be correctly followed by either a singular or a plural verb."
(Note the use of the word �may' by both...ie you need to consider the particular circumstances and it will depend on whether you are concentrating on the word's singularity or plurality aspect.)
These two publications have long been regarded as �bibles' of current English usage. Despite that, every time the question we are discussing here is asked and I pass on the views of the experts above, someone invariably appears and says exactly what the mistaken writer of the letter to The Times said...ie that one form of verb or the other is wrong.
Clearly, above I was concentrating on the army's plurality aspect. Cheers