I doubt Spencer fought in WW2, he would've been well over 50 by then, if anyone fought in WW2 it would be the baby (John II) that Alex had for him, he would've been 18 when the US entered WW2. So maybe Spencer's son loses his leg in the war, but still comes home and runs the ranch after Spencer's death in the late 60s until John III (Costner) takes over probably in the 90s or 2000s when the current young adult children on Yellowstone were growing up, its implied he was already in charge when his wife died but John II (baby at end of 1923, who should have 1 leg)) was still alive??? 1944 should tell us

One of which and I'm assuming it's the son Alex bore him is John Dutton on the present day Yellowstone father. John spoke of times he shared with his grandfather and Spencer lived to be very old, so I'm assuming Spencer is John grandfather and the premature baby grew into his father. Spencer and then his son born from Alex must have been the ones that kept the ranch afloat and growing from the 1920s or after Jacob dies until the mid/late 2000s when John II (baby from 1923) dies and John III (Costner) fully takes over.