This is precisely what was happening to me - my units were losing half their health making it to the city, and then had to fight a garrison unit when getting there. I changed my tactics and approached the city from a different direction where the terrain was more favourable and I was able to reach and capture the city in one turn. Happy Days.
Ackoman
I'm glad your issue was resolved!
Your quandary, however, has got me to wondering as to how some of my units expired even though I made it in 1 turn, and out the next. To answer this I ran 2 studies and arrived at the conclusion that strength of the attacking unit definitely has an effect when resolving the damage from taking a city.
Study #1
Nuke Kul Oba
35 melee strength Warrior Monk moves and takes Kul Oba
Pre-nuke Kul Oba wall strength = 53
Post nuke Kul Oba garrison rating = 49
Post-nuke warrior monk strength after taking the city = 57/100 health (so he should survive the turn).
Study #2
Nuke Chertomlyk
82 melee strength Helicopter moves and takes Chertomlyk
Pre-nuke Chertomlyk wall strength = 52
Post nuke Chertomlyk garrison rating = 44
Post-nuke Helicopter strength after taking the city = 95/100 health (he also should survive the turn).
So what we do know is that the melee strength of the attacking unit has bearing. If the pattern from the game holds true here as well, the city defense rating also has an effect. Therefore, I would surmise (although have not done any direct evidence) that later era (especially information age) opponents with stated garrison strengths significantly higher than Kul Oba’s post-nuke rating of 49 and Chertomlyk’s post-nuke rating of 44 would negatively impact the health %’s of the Warrior Monk (57/100) and Helicopter (95/100) perhaps below the 50% threshold, resulting in a casualty after the 50% radiation damage is applied.
Unfortunately, Sythia is the only civ left in my test and none of them advanced much past industrial anyway. I don't have any other saves with nukes and ran my present game out for that explicit purpose. So as to what would happen with information later era opponents? Who knows for sure. Anyone have a save? I'd like to find out for sure.
Here is the save, for anyone with the patience for a more detailed investigation.
*PS, don’t nuke Pokrovka the Macedonian unit will heal immediately after taking a city with a wonder.