what is segius opinion about soldiering and the secret of successfuling fighting?

I think this depends on where in the play you are referring to. By act 2 Sergius views war in a cynical manner. He thinks that there is nothing heroic about war. People merely try to survive by taking advantage of the enemy when they can, "Soldiering is the coward's art of attacking mercilessly when you are strong, and keeping out of harm's way when you are weak."