This raises a larger, related point. There must be a presumption of continuity in the foreign policy of a great power if allies are to remain allied and if foes are to be deterred. Unpredictability may on occasion make sense as a tactic, but not as a strategy. The many departures introduced or threatened by the Trump administration (most recently extending to both the NAFTA agreement and the 2015 nuclear accord with Iran) create doubts as to U.S. reliability. This is not meant as an argument for standing pat in foreign policy. The world is changing and U.S. foreign policy must change with it. The argument, though, is that the international project should be a renovation based on the existing order, not a teardown.

It needs pointing out that to recognize the revival of great-power rivalry, a prominent theme of the recently released National Security Strategy, provides little in the way of policy guidance. Countering Russian or Chinese challenges is necessary but not sufficient; doing so will not position the United States to meet regional and global challenges to its interests. What is required is the forging of effective collective responses, if need be without Russia and China, when possible with them.

The question naturally arises as to whether such a world could come about without the leadership of the United States, and in particular without the enthusiastic backing of the president and the executive branch. An optimist would argue that it could, that others in the United States and around the world would take up the slack. Alas, such optimism is mostly unwarranted.

It is true that Congress can do some things, such as introduce sanctions. It can also reject appointments, review treaties, hold hearings that shape public opinion, and withdraw or add funding. In addition, states and cities can do a good deal to offset executive disinterest in adopting policies to slow and adapt to climate change.

When all is said and done, though, the reality remains that in the American political system, most of the initiative when it comes to foreign policy lies with the executive. Critical positions (such as that of national security adviser) do not require confirmation, and the most important international agreements tend not to be in the form of treaties, in part to circumvent the need for Senate approval. Presidents have enormous latitude to use military force, to enter into and withdraw from negotiations and agreements, and to shape policy across the board, including the realms of both trade and immigration. It is difficult for Congress to restrain the executive—and even more difficult for it to compel the president to act or take the initiative when he holds back.

An optimist would also hope that other countries would pick up where the United States left off in promoting international order. The fact is, though, that there is no alternative great power willing and able to step in and assume what has been the U.S. role. China is often suggested, but its leadership is focused mostly on consolidating domestic order and maintaining artificially high rates of economic growth, lest there be popular unrest. China’s interest in regional and global institutions (including both its regional trade mechanism and its “One Belt, One Road” infrastructure initiative) seems more designed to bolster its economy and influence than to help set rules and arrangements that would be broadly beneficial. China’s assertiveness in the South China Sea (including its rejection of an international legal ruling challenging its claims) and its unwillingness to do all it could to rein in North Korea casts further doubts as to its readiness to fill the shoes of the United States.