PARIS — It took almost two weeks for negotiators from 195 countries to finally pass the landmark climate accord this weekend after several espresso-fueled all-nighters and long, passionate debates over the meaning of a single word, such as “shall.”

But the story of how the deal came together started long before that — in December 2009, with the failure of the last such summit meeting, in Copenhagen.

That gathering was, in hindsight, a case study in how not to do a deal. The hosts of the event had set a stern tone, with concrete barricades, concertina wire, and steel cages to house protesters who stepped out of line.