Is it obligatory to send a thank you message to people who attended your wedding?

I don't think its obligatory. It might be a nice gesture to send a card to those who signed a guest book. I also think thank you cards would be highly suggested if they gave a gift.

Otherwise 300 people may be too many to keep track of and send cards to.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.