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?
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?