how can nato be a constraint …

The constraints on American hegemony today are as given below:

(i) Institutional architecture of the American state : There is Presidential form of government in USA. Under this a system of division of powers and checks and balances between the three branches of the government limit the unrestrained and immoderate exercise of America's military power by the executive branch.

(ii) Open nature of American society: There is deep sckepticism regarding the purposes and methods of government in American political culture. This put a constraint on US military action overseas as had happened in case of Vietnamese war.
(iii) NATO: This organisation can moderate the exercise of American power because the US has an enormous interest in keeping the alliance of democracies that follow the market economies alive.

The constraint- i.e., institutional architecture of the American state is expected to get more important in future because the American constitution has not changed much during the last about two hundred years. The features of division of power etc. have remained intact. Nature of the people or the NATO may change in future. So the institutional structure will become more important in future.