Question: What was the earliest war to use both the M18 and the M1917?

Context paragraphs:
M1917 Enfield: The M1917 Enfield, the "American Enfield", formally named "United States Rifle, cal .30, Model of 1917" was an American modification and production of the British .303-inch (7.7 mm) P14 rifle (listed in British Service as Rifle No. 3) developed and manufactured during the period 1917–1918.  Numerically, it was the main rifle used by the American Expeditionary Forces in Europe during World War I.  The Danish Slædepatruljen Sirius still use the M1917, which performs reliably in Arctic conditions, as their service weapon.

M1917 Browning machine gun: The M1917 Browning machine gun is a heavy machine gun used by the United States armed forces in World War I, World War II, Korea, and to a limited extent in Vietnam; it has also been used by other nations.  It was a crew served, belt-fed, water-cooled machine gun that served alongside the much lighter air-cooled Browning M1919.  It was used at the battalion level, and often mounted on vehicles (such as a jeep).  There were two main iterations of it: the M1917, which was used in World War I; and the M1917A1; which was used thereafter.  The M1917, which was used on some aircraft as well as in a ground role, had a cyclic rate of 450 rounds per minute; the M1917A1 had a cyclic rate of 450 to 600 rounds per minute.

M18 Hellcat: The M18 Hellcat (officially designated the 76 mm Gun Motor Carriage M18 or M18 GMC) was an American tank destroyer of World War II, used in the Italian, European, and Pacific theatres, and in the Korean War.  It was the fastest American tracked armored vehicle until the turboshaft-powered M1 Abrams main battle tank appeared decades later.  Even then, most sources list the M1 Abrams with a top speed of only 45 (governed speed) mph, leaving the Hellcat with a superior paved-road top operating speed.  The speed was attained by keeping armor to a minimum, no more than one inch thick and roofless, and by powering the relatively small vehicle with a radial engine originally designed for aircraft usage.  The Hellcat, along with the M4 Sherman-based M10 tank destroyer and the highly effective, 90mm gun-armed M36 tank destroyer, provided American and Allied forces with a mobile anti-tank capability against the newer German armored types.

M18 recoilless rifle: The M18 recoilless rifle was a 57 mm shoulder fired anti-tank recoilless rifle used by the U.S. Army in World War II and the Korean War.  Recoilless rifles are capable of firing artillery-type shells at reduced velocities comparable to those of standard cannon, but with greater accuracy than anti-tank weapons that used unguided rockets, and almost entirely without recoil.  The M18 was a breech-loaded, single-shot, man-portable, crew-served weapon.  It could be used in both anti-tank and anti-personnel roles.  The weapon could be both shoulder fired or fired from a prone position.  The T3 front grip doubled as an adjustable monopod and the two-piece padded T3 shoulder cradle could swing down and to the rear as a bipod for the gunner.  The most stable firing position was from the tripod developed for the water-cooled Browning M1917 machine gun.

Fusil Automatique Modèle 1917: The Fusil Automatique Modèle 1917 (also called the RSC M1917) was a semi-automatic, gas-operated, infantry rifle that the French Army placed in service during the latter part (1918) of World War I.  It was chambered in then-standard 8mm Lebel, the rimmed cartridge used in other French Army infantry weapons of the time.  Altogether, MAT (Manufacture d'Armes de Tulle) had manufactured 86,000 RSC M1917 rifles when their production run ended in late November 1918.  However very few have survived in fully functional, semi-automatic condition and those have become highly sought-after collectibles.

M1917 revolver: The M1917 Revolver (formally United States Revolver, Caliber .45, M1917) was a U.S. six-shot revolver of .45 ACP caliber.  It was adopted by the U.S. Army in 1917 to supplement the standard M1911 .45 ACP semi-automatic pistol during World War I. Afterwards, it was primarily used by secondary and non-deployed troops.  There were two variations of the M1917, one made by Colt and the other made by Smith & Wesson.

AN M18: The M18 Colored Smoke Grenade is a US Army grenade used as a ground-to-ground or ground-to-air signaling device, a target or landing zone marking device, or a screening device for unit maneuvering.  The M18 replaced the World War II M16 which did not burn as long or as vividly.  Both were produced at the same time as the M16 production line was already setup when the M18 was adopted.  The M16 was available in the same colors as the M18, but also blue, orange, and black.  The M18 initially were going to be produced in the same colors including white, but it was decided to limit it to four colors for simplicity.  It is not designated the "AN-M18" but "M18."  It can be seen during the "Purple Haze" scene, in the film "Apocalypse Now" (1979).  It was discovered in Vietnam that the green smoke drove away swarming bees.  When a ground element popped smoke to identify its location to aircraft, the aircraft was not told the color, but told to identify the color they saw.  This prevented the VC/NVA if monitoring the frequency from popping smoke of the same color to confuse the aircraft.  For additional security the colors were sometimes identified as cherry (red), lime (green), lemon or banana (yellow), or grape (violet).

Brodie helmet: The Brodie helmet is a steel combat helmet designed and patented in London in 1915 by John Leopold Brodie.  In modified form it became the Helmet, steel, Mark I in Britain and the M1917 Helmet in the U.S. Colloquially, it was called the shrapnel helmet, Tommy helmet, tin hat, and in the United States the doughboy helmet.  Worn by Australians during WW2 and sometimes known as Panic Hat.  It was also known as the dishpan hat, tin pan hat, washbasin, battle bowler (when worn by officers), and Kelly helmet.  The US version, the M1917, was copied from the British Mk 1 steel helmet of 1916.  The German Army called it the "Salatschüssel" (salad bowl).  The term "Brodie" is often mis-used.  It is correctly applied only to the original 1915 "Brodie's Steel Helmet, War Office Pattern".

M39 Armored Utility Vehicle: The M39 Armored Utility Vehicle (T41) was an American armored vehicle designed during the Second World War, which saw service in that conflict and in the Korean War.  Like a number of vehicles of this type, it was built using an existing chassis, that of the M18 Hellcat.

Remington Model 30: The Remington Model 30 is a US sporting rifle of the inter-war period based on the military P14/M1917 Enfield rifle action, which was manufactured for the British and US governments during World War I. Initial specimens used surplus military parts with some modifications in order to consume the stock of parts, though further modifications were made as production progressed and later rifles were produced from newly manufactured parts.  Most early rifles were in the military .30-06 calibre used in the M1917 but it became available in a variety of chamberings.  It was the first high-powered bolt-action sporting rifle produced by Remington.

Supporting paragraphs: M18 recoilless rifle M1917 Browning machine gun
Answer: World War II