In formal writing (MLA format essay) when should you capitalise the word colonies? I imagine the in the phrase "The Thirteen Colonies" it would be as that is a title, but I'm not sure if it is indeed correct. Also would it be capitalised in a sentence like one I'm writing, "This caused a rise in Christian influence in the Colonies" is that correct? Or should it be lowercase? Thanks.
I don't think that in the sentence you quote that I would capitalise colonies.
There are instances, where, for example, if I was referring to them as an entity, I might use capitals - 'The Thirteen Colonies', or even 'The Colonies'.
e.g. The population of Britain at the time was about 9 million, France 25 million and The Thirteen Colonies about 4 million.
But there are other instances, where if, for example, I was just writing about British expansion generally I might say something like -
...Britain's influence in North America following the Seven Years' war with France, was at its peak - across the whole of Canada and in the thirteen colonies of Virginia, Maryland, New York, Connecticut, New Jersey etc.