It doesn’t seem like knowing your enemy and knowing yourself should actually make you invincible in war. Besides, what if your enemy also knows themselves and knows you?
It makes more sense if you consider that another option is to avoid the war. So I would interpret it like this:
If you know that you are strong and that the enemy is weak, you will win the war. (And if you know otherwise, you will avoid the war—by keeping peace, paying tribute, or surrendering.)
If you know that you are strong, but you don’t know your enemy… sometimes you will win, sometimes you will be surprised by finding that your enemy is strong, too.
If you have no idea, and just attack randomly… expect to get destroyed soon.
In this light, the next quote would be interpreted like: before you start the war, make sure to build a strong army, so that you don’t have to improvise desperately after the war has started.
It makes more sense if you consider that another option is to avoid the war. So I would interpret it like this:
If you know that you are strong and that the enemy is weak, you will win the war. (And if you know otherwise, you will avoid the war—by keeping peace, paying tribute, or surrendering.)
If you know that you are strong, but you don’t know your enemy… sometimes you will win, sometimes you will be surprised by finding that your enemy is strong, too.
If you have no idea, and just attack randomly… expect to get destroyed soon.
In this light, the next quote would be interpreted like: before you start the war, make sure to build a strong army, so that you don’t have to improvise desperately after the war has started.