I saw this sentence and thought shouldn't "will" be placed after "You" instead of before it?
Only by defending against your opponent's threats will you be able to successfully carry out your own strategies.
Sometimes only means 'not - except in this situation'. If we use it to modify an Adjunct at the beginning of a sentence, we need to change the order of the Subject and the auxiliary verb in the main clause. We need the same word order that we find in questions:
If we don't do this the sentence is ungrammatical:
An Adjunct is a word or phrase that gives you extra information in a clause or sentence. It isn't a Subject or a Complement of the verb (for example an Object). An Adjunct is not essential for the grammar. It is extra information that we usually put at the end of the sentence in English. Adjuncts can be preposition phrases, adverb phrases or noun phrases. In these examples the Adjuncts are in bold letters:
Notice that if we want to we can put the Adjuncts at the beginning of the sentence:
Now, if we want to say that the thing in the main clause only happens in the situation described in the Adjunct, we can use the word only. The normal place to put this word is after the first auxiliary verb. If there is no auxiliary verb, we put it before the main verb:
Now suppose we want to move this adjunct to the front of the sentence. Let's try:
This sentence is grammatical, but it means something very different. The problem is that the word only nearly always emphasises something in front of it. It can't normally emphasis something earlier in the sentence. The first sentence means that she won't do anything else, she will only offer to help. We wanted it so say that she will offer to help but only when she understands about the serious situation. The second sentence means they only told me the news, they didn't do anything else. We need it to say they told me the news, but only recently and not before.
So, we can fix this if we put the word only directly before the Adjunct. So for example we can write Only when she understands the situation .... Let's try it:
Ooops. That's not a good result. This sentence is ungrammatical. The word only has several meanings. We are interested in the one which shows that something doesn't happen except in this situation. When we put this only before an Adjunct at the beginning of a sentence, we need to change grammar in the main clause. We need to invert the Subject and the auxiliary verb. We need to put the auxiliary before the verb:
If the sentence was in the present simple or past simple, there will not be an auxiliary, we will need to use the dummy auxiliary DO, like we do in negatives and questions:
Here are the other examples, this time with [only + Adjunct] at the beginning:
The Original Poster's example:
Here only comes before an Adjunct at the beginning of the sentence. We need subject-auxiliary inversion here. The will needs to come before the you:
[Only by defending against your opponent's threats] will you be able to successfully carry out your own strategies.
I think you can put the auxiliary will after you in informal English.
However, in formal and written English, you can put will before you.
According to The Free Dictionary, in writing and formal speech, you can use "only" at the beginning of a sentence, followed by a word, phrase, or clause it applies to. After this word, phrase, or clause, you put an auxiliary verb or be followed by the subject of the main clause. Some examples are given below:
Only in the Southwest do you find scenery like this.
Only here do I feel safe and secure.
Only then did I realise that I was too late.
In your example, will is switched with you to achieve emphasis on the conditional - only
Only by doing something will you be able to do something else
consider the alternatives
By doing something you will be able to do something else
If you do something you will be able to do something else
You will be able to do something else only by doing something
The emphasis of doing something as the solution (only) is much stronger in the first sentence than the others
Only at the beginning the sentence makes the structure idiosynchratic, consider
If you only do something you will be able to do something else
If you were only to do something you will be able to do something else
The emphasis in your sentence is on the defensive, however
The best defence is a good offence
To avoid the split infinitive in your original sentence
Only by ... will you be able to carry out your own strategies successfully.
Only by ... will you successfully be able to carry out your own strategies.