The usage of word that can be both plural and singular

Question

Words such as Infantry and Cavalry according to Oxford Learner's Dictionary can be both singular and plural.

Here is a specific example from their site.

The infantry was/were guarding the bridge.

So I can choose either was or were. My question is whether the meaning is exactly the same. If there is a slight difference in meaning or nuance could you please explain what it is?

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Another example:

Five hundred infantry was marching to the north.

Five hundred infantry were marching to the north.

Five hundred infantries were marching to the north.

Are they have the same meaning?

Answer 1

(This answer describes British usage: American usage may be different).

For collectives generally, if what you are saying is true of the collective as a whole, then use the singular: The infantry is made up of many separate units.

If what you are saying is true of the individuals in the collective, then British usage prefers a plural: The infantry are advancing.

Sometimes what you are saying will be true of both the whole and the individuals; then British usage again prefers a plural: The infantry are a proud and ancient part of the army.

If you use an explicit number (greater than 1, of course, but I can't image "one infantry" being used), then it's plural 500 infantry are advancing.