If a player sends reinforcements to your city and you want to shield yourself, then once you activate the Shield of Peace, all allied reinforcements will be sent back to allies.
You cannot send reinforcements to a player protected by the Shield of Peace: your reinforcements will turn back as soon as they reach the shielded city.
You can send reinforcements to non-shielded players while (yourself) protected by the Shield of Peace. However, this will not allow you to score points in the Clash for Thrones.
You cannot activate the Shield of Peace when you've sent reinforcements to allied cities or even if you have active marches towards allied/enemy cities.