This is, it should be said, less true on foreign policy than on domestic policy. But on domestic policy, it's quite true. If you could convince Barack Obama that the economy desperately needed $400 billion more in immediate stimulus, there's just not that much he could do about it. He doesn't have the votes, and like all presidents before him, he doesn't have a irresistible powers or tools of persuasion he can use to get the votes. Our system is based around Congress even if our storytelling is based around the president.Well...sort of. It is true that the president has a Constitutional advantage with respect to the Pentagon that he doesn't have over the rest of the government. Beyond that, however, I think that the president's influence in foreign policy is often an illusion. It certainly isn't absolute.
Consider. George W. Bush didn't come into office wanting a war in Afghanistan...as far as I can tell, he (and his neocon supporters) wanted to ignore bin Laden, and the Taliban, as much as possible. Obviously, that lasted until the morning of September 11, 2001, after which any future efforts he made to ignore bin Laden proved fairly costly.
Or, consider Iraq. Yes, Bush got his war there. But first, note that at least in my opinion it's an open question as to whether Bush wanted that war or not. I think there's plenty of evidence that the neocons wanted the war in Iraq, but it's not clear to me whether they manipulated Bush into it or if Bush wanted it from the get-go. Remember, we're talking here about the president, not about the government in general. Supposing, however, that Bush did want the war...well, yes, he did get it. However, the things he needed to do to get the war doomed it to failure. He couldn't deploy half a million troops and promise to keep them there for a few years; he couldn't give an honest reckoning of what the United States actually knew about Iraq's weaponry; he couldn't give weapons inspectors time to do their work. Had he done those things, he would have risked losing support in Congress and with allies abroad. So he didn't, and as it turned out that had costs, too.
Even when the president appears to have the strongest lines of authority his influence can be very limited. I'm pretty sure that Obama is going to get DADT repeal -- but only because, unlike Clinton, he's going about it by building support carefully, instead of by just assuming he can give orders and they'll be carried out without any negative consequences. He's having some success in procurement reform, too, but against that's hard work, fraught with dangers for the president. (And in both cases, not just because Congress can get involved -- but part of the idea here is that Congress can always get involved).
Think too about EPA carbon regulation. The EPA seems to have fairly broad powers. But that doesn't mean that Obama can simply give orders and assume they'll be carried out and that will be the end of it. He has to worry about further court decisions; about Congressional actions to overturn what EPA does; and even the possibility that the bureaucracy at EPA won't like his decisions, and will fight back against them using leaks to threatened interest groups and their Congressional allies.
Getting back to foreign affairs...yes, it's often true that he has fewer domestic constraints, although as the above examples show that's not always true. On the other hand, the president probably has more ways to influence Ben Nelson and Olympia Snowe than, say, the governments of Britain and France, not to mention the governments of Iran and North Korea. And in both domestic and foreign affairs, events that the president simply can't affect -- oil spills, terrorist attacks, hurricanes, flotillas -- can change things and "force" actions that were far from the president's original plans.
Of course, the president isn't powerless. It's just that, as Neustadt explains, he has to build his influence, and use it carefully. The details change, and some situations make it easier and others harder, but in general it's true in foreign affairs, just as it is in domestic affairs.