Shortly after the German sinking of RMS Lusitania, the US government fired off the following memorandum to the German government:

In view of recent acts of the German authorities in violation of American rights on the high seas which culminated in the torpedoing and sinking of the British steamship Lusitania on May 7th 1915, by which over 100 American citizens lost their lives, it is clearly wise and desirable that the government of the United States and the Imperial German government should come to a clear and full understanding as to the grave situation which has resulted.

The sinking of the British passenger steamer Falaba by a German submarine on March 28th, through which Leon C. Thrasher, an American citizen, was drowned; the attack on April 28 on the American vessel Cushing by a German aeroplane; the torpedoing on May 1st of the American vessel Gulflight by a German submarine, as a result of which two or more American citizens met their death; and, finally, the torpedoing and sinking of the steamship Lusitania constitute a series of events which the government of the United States has observed with growing concern, distress, and amazement.

The government of the United States has been apprised that the Imperial German government considered themselves to be obliged by the extraordinary circumstances of the present war and the measures adopted by their adversaries in seeking to cut Germany off from all commerce, to adopt methods of retaliation which go much beyond the ordinary methods of warfare at sea, in the proclamation of a war zone from which they have warned neutral ships to keep away. [The US] government has already taken occasion to inform the Imperial German government that it cannot admit the adoption of such measures or such a warning of danger to operate as in any degree an abbreviation of the rights of American shipmasters or of American citizens bound on lawful errands as passengers on merchant ships of belligerent nationality; and that it must hold the Imperial German government to a strict accountability for any infringement of those rights, intentional or incidental.

It does not understand the Imperial German government to question those rights. It assumes, on the contrary, that the Imperial government accept, as of course, the rule that the lives of non-combatants, whether they be of neutral citizenship or citizens of one of the nations at war, cannot lawfully or rightfully be put in jeopardy by the capture or destruction of an unarmed merchantman, and recognize also, as all other nations do, the obligation to take the usual precaution of visit and search to ascertain whether a suspected merchantman is in fact of belligerent nationality or is in fact carrying contraband of war under a neutral flag…

American citizens act within their indisputable rights in taking their ships and in traveling wherever their legitimate business calls them upon the high seas, and exercise those rights in what should be the well justified confidence that their lives will not be endangered by acts done in clear violation of universally acknowledged international obligations, and certainly in the confidence that their own government will sustain them in the exercise of their rights…

The government and the people of the United States look to the Imperial German government for just, prompt, and enlightened action in this vital matter with the greater confidence because the United States and Germany are bound together, not only by special ties of friendship but also by the explicit stipulations of the treaty of 1828 between the United States and the Kingdom of Prussia.

Expressions of regret and offers of reparation in the case of the destruction of neutral ships sunk by mistake, while they may satisfy international obligations, if no loss of life results, cannot justify or excuse a practice, the natural and necessary effect of which is to subject neutral nations and neutral persons to new and immeasurable risks.