Just Cause or right intention: The sinking of this ship was not for the aim of peace and was purely for personal interest. Germany justified the attack by saying that the ship was an enemy ship so they have every right even though there were many innocent citizens on the ship at the time.
Last resort: All nonviolent options were not tried before they sunk the ship. They had no actual reason to sink the ship and it not contribute to their success in any way.
Probability of Success: If anything, attacking the Lustiana lowered their chances of winning the war. Their goal was to attack the ship since it was armed and that although achievable, did nothing to help them.
Proper authority and Public Declaration: No one expected this attack and it was completely uncalled for. No legal authority called to do this and a military leader took it upon themselves to do this.
Proportionality: This violence was not proportional to military objections. The ship contained mostly innocent civilians and the military attack killed all of them for no actual reason.
JUS AD BELLUM
Military Necessity: Attacks should be to defeat the enemy not to attack ships becase they are armed. Germany did not try to limit death and destruction when they sunk the entire ship.
JUS IN BELLo
No means malum in se: There were no nuclear or chemical weapons but I think sinking an entire ship full of innocent civilian goes against this criterium.
Fair treatment of Prisoners of War: For this particular event, there were no prisoners of war involved.
Distinction: This attack directly attacked many civilians and sunk the entire ship including their personal property and family.
Right Intention: Germany did not have the right intention when they attacked the ship and they took revenge for the sole reason that the ship was armed.
Discrimination: This punitive measure directly affected citizens and not the military leaders or whoever was responsible for arming the ship.
JUS POST BELLUM
This was not a just event
Civilians were harmed and punished in this event even though it clearly states in all three of the criteria that citizens should not be harmed or punished in war. Therefore, the sinking of the Lusitania was not a just action in this war.