Warriors are needed because as long as power exists, there will be struggles. In fact, someone, somewhere, right now is willing to harm the US because of the ideals they believe this nation possesses.
Many times diplomacy and negotiations take place, but they do not always resolve conflicts. Diplomacy and negotiations usually only appeal to logical and rational people. When these actions do not work or if the enemy is illogical, irrational and only responds to fear, the US may have to use military force.
Warriors are also an essential part of any peaceful civilization; they hunt evildoers and stand watch over society. The US could not have peace without warriors. They must bear arms to protect the US and its citizens.
Without warriors, other nations or groups with power would more easily prey on the US. Warriors must be prepared to act at all times. While citizens believe no harm can ever come to them and often practice “if/then thinking (if this happens, then I will do this),” a warrior remains vigilant for danger and practices “when/then thinking.”