With only hours left to go on the final day of the talks, American diplomats noticed a discrepancy in the new climate agreement’s text. Where previous drafts of the pact had said that rich countries “should” take the lead on preparing greenhouse-gas reductions plans, the final draft replaced that word with the more definitive “shall.” If the new treaty had created new binding legal requirements for the United States, President Barack Obama would have had to submit it to the Republican-controlled Senate, which would have surely rejected it. After some tense bargaining, a revision was rushed through at the last moment.