Memorial Day will someday mean a double mourning, a mourning for the violent deaths suffered by of millions of American military and a much more agonizing mourning for the deaths, maiming, destruction and suffering these Americans in uniform brought to millions of innocent men, women and children by committing crimes against humanity in their own beloved countries for the lies of their government and media.
Regarding any order to invade and or kill in another country: "An order which is unlawful not only does not need to be obeyed but obeying such an order can result in criminal prosecution of the one who obeys it. Military courts have long held that military members are accountable for their actions even while following orders -- if the order was illegal.[U.S. Military Careers " U.S. Military Justice System, Oct 18, 2016] 
"I was only following orders," has been unsuccessfully used as a legal defense in hundreds of cases (probably most notably by Nazi leaders at the Nuremberg tribunals following World War II). The defense didn't work for them, nor has it worked in hundreds of cases since.
Here below copied are the very clear Nuremberg Principles of International Law, which former US Attorney General Ramsey Clark unequivocally states are part of the law of the land by Article Six of the US Constitution.
"Any person who commits an act which constitutes a crime under international law is responsible therefore and liable to punishment."
"The fact that an internal law does not impose a penalty for an act which constitutes a crime under international law does not relieve the person who committed the act from responsibility under international law."
"The fact that a person who committed an act which constitutes a crime under international law acted as Head of State or responsible government official does not relieve him from responsibility under international law."
This paraphrases to, "even if you are head of state you still can be tried under international law."
The fact that a person acted pursuant to order of his Government or of a superior does not relieve him from responsibility under international law, provided a moral choice was in fact possible to him".