If we think of a shield we think of it being used for protection, but a shield is designed to be used in battle. In essence it is an instrument of war. In life we often use words and actions as a shield. We might tell a joke or become defensive in order to protect ourselves in some way. The assumption when doing such a thing is that we are in conflict with the world.
I would argue that being in conflict is a state of mind that comes out of the thoughts that we have. It is all tied up in our identity, our past experiences, how we grew up, the relationships we have and have had, etc, but in the present moment it is controlled by our thoughts. If we change our thoughts we change how we interact with the world. If we stop feeling like we are in conflict with the world we will stop needing our defences and we can live more in harmony with others.
To live in peace you must first have a peaceful mind. This can be difficult to achieve but it can be done by doing the work, through meditation, counselling, self analysis and spiritual exploration. In the end we are responsible for how we are in the world and how we treat others and ourselves.