What is a War Nickel?

By GovMint :

War Nickels are Jefferson five-cent coins produced from 1942 to 1945. During World War II, nickel was a critical material needed for the war effort, so the U.S. Mint altered the composition of these coins to include 56% copper, 35% silver, and 9% manganese. Congress authorized this change to conserve nickel for military use. These coins are easily identifiable by their large mint marks ("P," "D," or "S") located above Monticello on the reverse side. War nickels also notably marked the first time the "P" mint mark was used on U.S. coins. These nickels have a distinct appearance, often darker with a greenish tint due to the silver content.

Sources Used:

https://coinweek.com/war-nickel-how-to-identify-these-valuable-nickels/