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September 4, 1917: the American Expeditionary Forces in France suffered their first fatalities during World War I when a German plane attacked a British-run base hospital in Camiers. There was an unexpected windfall for the Allies when America joined the war—help with the enormous load of casualties in the 1917 campaign. The first American military installation in France during World War I was Base Hospital No.4 (Cleveland), which arrived on 25 May 1917 to cheering French crowds on, 19 days ahead of General Pershing and the nucleus of his American Expeditionary Force staff. Also known as the Lakeside Unit, the Cleveland unit served at Rouen throughout the war. American physicians, nurses, and enlisted men such as these would be the earliest AEF participants to face the possibility of death and destruction—actually months before the first American soldiers would see combat. Base Hospital No.4 treated 82,179; No. 10 treated 47,811; and No. 21 treated about 60,000. These numbers do not include the numbers of patients the Americans treated at the Casualty Clearing Stations or while working with British units. Overall, a daily average of approximately 800 officers, 600 nurses, and 1,100 soldiers was serving with the British. One of the hazards they faced was German night bomber raids, which attacked hospitals despite the Red Cross markings. Tragedy struck on 4 September 1917, when the Germans bombed Base Hospital No. 5 at Camiers, killing Lieutenant William T. Fitzsimmons and Privates Oscar C. Tugo, Rudolph Rubino, Jr., and Leslie G. Woods, who became the first A. E. F. casualties by enemy action. http://www.ourstory.info/library/2-ww1/hospitals/bh5a.html

To the uninitiated reader, it may be of interest to know how the approach of the enemy planes is detected. Every one has heard at some time the droning of a gasoline or electric motor and is aware that different motors have different vibrations. Allied and enemy planes were accordingly tuned differently. At several points in the rear of the lines there were tuning forks adjusted to detect the vibration of enemy motors. As soon as a plane came within range the tuning fork would pick up the vibration, and the warning would be relayed immediately to points farther in the rear.
On the evening of September 3 an attempt by the Germans to raid the English coast was made but it was successfully repulsed by the coast defense guns. The following day, about noon, a photographic scouting plane came over our area. The weather was clear and the air crisp, thus making ideal conditions for photographic work. The anti-aircraft guns, both mobile and stationary, did valuable work in keeping the plane at a very high altitude. Some criticism was heard concerning Colonel Patterson's desire to have the Stars and Stripes flying over the hospital area from such a tall flag-pole. It is believed by some to be possible that a photograph of this camp, showing the United States flag flying overhead, was secured and that, as a warning to other Americans who were to follow us into the battlefields of France, we should be made the objective of an aerial attack in spite of the fact that we were a hospital unit and therefore classified as non-combatants.
Be that as it may, we know not what was in the minds of the Germans when, in the evening of September 4, they dispatched their mission of death, having as their objective an American Base Hospital caring for two thousand sick and wounded patients.
There had been an attempted raid on the English coast earlier in the evening which had apparently failed. At ten-thirty we received a warning that enemy planes were approaching along the coast. The anti-aircraft guns at Sainte Cecile Plage and at Neufchatel were actively employed for a few minutes but were soon quiet and the all-clear was sounded. At ten fifty-five P.M., without any warning whatsoever, and while all lights in the vast twelve thousand bed hospital area were illuminating the camp, an enemy aeroplane suddenly swooped down over the brim of the circle of high hills from the direction of Etaples. A few minutes prior to this incident a loud report as of the crashing of a bomb had been heard from that direction, but by those who had heard it, it was mistaken for the report of an anti-aircraft battery. Lieutenant William Fitzsimmons, who had recently been appointed adjutant of Base Hospital No. 5, was among those who heard the first report, and fearing the possible approach of enemy planes had summoned the sentry, Private Hiram Brower, to ascertain the cause of the violent explosion. Having answered the question of the adjutant, the guard resumed the patrol of his post. Scarcely a minute had elapsed when another more violent explosion occurred, caused by the dropping of an aerial torpedo on General Hospital No. 18. Fortunately no damage was done as it dropped in the center of the athletic field tearing a deep hole several yards in diameter. Then swinging his plane in a semicircular course, a bomb of the smaller type was dropped into the reception tent of No. 4 General Hospital, followed almost immediately by two bombs that dropped within eighteen inches of each other in front of Lieutenant Fitzsimmons' tent, two others at each end of Ward C-6 and another in the reception tent of No. 11 General Hospital. Lieutenant William Fitzsimmons was instantly killed by the first two bombs to be dropped on Base Hospital No. 5, while the flying fragments wounded Lieutenants Rae Whidden, Thaddeus Smith, Clarence McGuire and Private Hiram Brower. Fragments from the two bombs which were dropped on Ward C-6 killed Private Oscar Tugo and several patients, while other patients were wounded and large portions of the ward were wrecked.

<first American graves in France. Some very cool photos: https://www.theatlantic.com/projects/world-war-i-in-photos/