As mentioned above, my dad served in the Army National Guard at the very end of Korea. He stayed on active duty with the Guard for a couple of years but was in Detroit the rest of the time. At the very end of his active time his unit was activated to be sent to Egypt for the Suez Canal Crisis but they just sat around on standby in a warehouse here for a week and never actually went.
He stayed in the Guard on reserve duty for about another decade and when his unit got orders to augment an Army artillery unit in Vietnam he checked in at the armory and was told he was too old and had too many kids and they wouldn't take him so he ended his reserve enlistment shortly after that.
My FIL was a Navy officer during the early days of of US involvement in Vietnam and served with the "brown water navy" and was part of the putting together the Mobile Riverine Force. My wife says he never talked about his time in the Navy when she was growing up but he's shared some stories with me and from what he's told me and from what I've read from other accounts from the MRF, other than being a Navy Corpsman attached to a Marine unit, it was probably the most dangerous job in the Navy at the time.
Neither of my grandfathers served in the military during WWII: my paternal grandfather was too old and my maternal grandfather had had horrible TB and also couldn't see sh!t. Most of my dad's uncles joined the Army right after Pearl Harbor and all were sent overseas; three went to Europe (one served as a baker, one was in the infantry, and the other was a paratrooper) and one other of his uncles joined the Navy and became a Seabee and spent most of his time building runways and setting up camps in the Pacific. My great uncle who was the paratrooper was a Ranger and his unit was dropped behind German lines a few weeks before D-Day with the goal of setting up some reconnaissance or something. At some point, their radioman turned on some comm equipment and ended up giving away their position. They were captured and he spent a few months as a POW. My dad can vaguely remember most of his uncles (all of them died young except the baker, who lived long enough for me to have a lot of fond memories of him) but does remember that the one that was the paratrooper and the one that was the Seabee both suffered badly from PTSD.
My dad's maternal grandfather served in the German Army before WWI and was some high-up muckity muck with the Thyssen Iron Works. He was somehow able to get his two youngest children (my oma and a great uncle) out of Germany and into America soon after the start of the war but before the U.S. got involved. They both were smuggled out through Spain or Portugal. A lot of Thyssen execs were basically lynched after the war and lost everything but my great grandfather's family managed to come through relatively unscathed. My oma had another brother and sister who stayed in Germany and after the war and after my dad was born my oma, her husband, and my dad moved to Germany for a couple of years. After a year or two, my grandfather thankfully convinced her to move back to the US and they did so in '38. My dad says he has some uncles that he's pretty sure served in the German Army during WWII but doesn't know for sure and whether or not they were conscripted or enlisted or what was going on as once they moved back to the US, my oma basically cut off ties with her family for decades.
The furthest back we've been able to trace military service in our family is a great great or great great great grandfather on my paternal grandfather's side who served in a militia form North Carolina during the War of 1812. We were only able to verify his service thanks to most of their unit being captured and finding his name in old military prison camp records. Looking through the old records, it looked like at some point before the end of the war, the British just let all the prisoners at this particular camp go, gave them each something like $2 and sent them on their way.