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Ludology Host and Dice Tower Contributor
If a neutral nation is attacked and a quagmire results, can the opposing player reinforce it if he cannot trace a line of supply from his supply source to the province? Could he add cards to the province's defence before the attack if he cannot trace a line of supply?
Yes you can. Supply does not affect your ability to place a card defensively (including defending a quagmire).
Note that there in the Advanced Realism rules there is a rule you can add that does not allow defenders to be placed in out of supply provinces. I strongly recommend adding this rule once you've played a few games - we left it out as we found that it was confusing and usually forgotten by beginners.