Joined In crushing the rebels / joined into crushing the rebels

Question

The commoners joined the king's army at crushing the rebels.

Which is the best preposition to replace the bold part in the above sentence?

I had four options to choose from which I managed to rule out two, but I am confused between these two options:

A) in

B) into

We use into when we want to manifest motion e.g. I saw a snake going into the room and use in when there is no need to manifest motion e.g. The cat was sitting in the room. But which preposition fits in the above sentence? Going by the explanation which I gave, option A > option B, but my book has option B as its answer. I suspect it's an error in printing.

Answer 1

Your ideas are logically correct.

But to "join in" is an idiom, according to thefreedictionary.com, which has been useful to me, as a native English speaker, to pin down precise idiom meanings. "Join in" assumes intent or choice. "Join into" is more for strictly physically joining.

For instance,

• "We're eating ice cream. Would you like to join in?"
• My friends, who were racing to the park, let me join in.
• I joined my friends in eating ice cream.
• I joined my friends in racing to the park.

However, as you mentioned earlier, "join into" can be appropriate for physical joining, especially of two non-person subjects. This is different from "join in" where someone starts to act the same as others.

• The Missouri River joins into the Mississippi River north of St. Louis.
• East Germany was joined into West Germany in 1990.

Also, both "The two sides were joined into battle" or "The two sides were joined in battle" sound right to me, although "in" seems a bit better. This is an edge case I can't explain precisely why, but my rough explanation is that they could have done so intentionally or unintentionally, and they're more likely to have done so intentionally. But they could have been, say, forced to the same field by the weather.

Incidentally, the extra subject between join and in doesn't matter here, although you could change

"I joined my family in cooking dinner" with "I joined in with my family cooking dinner."

Though I prefer join (subject) in.

I think there may be another way to look at it, but the main things are:

• join in is an idiom
• join into is more strictly physical, without intent. For instance, in the above phrase, "The commoners joined the king's army in taunting the rebels." also works, but it involves less physical activity. So maybe you can also see if you can replace the more active motion verb with something less active, and if so, "in" is preferred to "into."

Answer 2

I'd caution you to be careful with generalizations such as "we use .. in when there is no need to manifest motion." That's true, but there are many other reasons to use in as well; prepositions are notoriously flexible and multifaceted in that regard.

If you go to Wordnik, you'll see over a dozen possible uses for the preposition in. The one I think fits best here is:

in (prep.) With the aim or purpose of : followed in pursuit

We could use a simple substitution to see how this works. First we plug in the definition (I've substituted the word aim with its synonym goal):

The commoners joined the king's army with the goal of crushing the rebels.

Change that bolded text to in:

The commoners joined the king's army in crushing the rebels.

Answer 3

The commoners joined the king's army in crushing the rebels.

IN is what you are looking for here.

For clarity this works too:

"The commoners, joined the king's army to help crush the rebels."