This one always causes problems for students, and some native speakers: "Why is it 'the army is' but 'the police are'?"

The reason is believing that police and police-force are near enough synonymous, but they are not.

The *police-force* is an organisation; *the police* are the members of the police force; and a *police officer* is one item of that group.

To compare this with the army, then *army* equates to *police-force* - not *police*. 'The police-force is' and 'the army is'. 

*Police* equates to *soldiers*. 'The soldiers are' and 'the police are'.