When it comes to war between two countries or a duel between two people it is always best for you to know how to find the weakness of your enemy.
If you are capable of finding out about your enemy's weakness, then you can rest assured that you have already won the war.
Just like a duel between two people, a war between two countries can also be won if one country can find out about the weakness about the other.
This is based on the simple fact that countries are ruled by humans and humans have weakness.
If one country can figure out the weakness of the other country's ruler, then the downfall of that particular country is inevitable.
At the smaller scale, you could either have somebody snoop on your enemy to find out what his weakness is or perhaps go by the saying "keep your enemies closer".
When you keep your enemy closer to you, you would know what his weakness is easily, without having to rely on the words of another person.
Once you have gotten to know about your adversary's weakness, just work on it and bring about his downfall.
If your adversary has a weakness towards alcohol, then let that be the best way to make him spill those beans that he isn't supposed to.
Even before working upon your enemy's weakness you first need to come up with a plan to find out what they are.
As stated before, you can use a friend to do the dirty but much needed work or perhaps do some extended research over the internet.
With the right answer on how to find the weakness of your enemy, even you can make your dreams come true.