Whether you know it or not, if you are a male between the age of 17-45 in the US, you are considered eligible to be called to militia duty by your state.

All able bodied men, 17 to 45 of age, are ultimately eligible to be called up into military service and belong to the class known as the reserve or unorganized militia (10 USC).

The Dick Act

https://en.wikipedia.org/wiki/Militia_Act_of_1903