As the Supreme Court hears a final day of arguments over the health-care law, there is widespread agreement that in Tuesday’s marquee matchup, U.S. Solicitor General Donald Verrilli—tasked with defending the law—had a rough go of it. But Verrilli is one of the most gifted advocates of his generation. So what happened?

To answer this it’s helpful to consult one of the oldest battle treatises still in print. In The Art of War, Sun Tzu defined a fundamental tenet of battle strategy: never fight on the terrain that favors your enemy. The health-care law’s challengers decided to frame this case as being about an unprecedented effort by the government to force the purchase of a product (in this case, health care). If that’s what they thought the best framing was, you can be sure it was not the best framing for the government. And yet the government inexplicably offered no alternative framing in the months leading up to this week’s showdown.

In fact, the challengers’ claim is completely false. In 1790, the very first Congress (which included 20 framers of the Constitution, in case Justices Thomas and Scalia are counting), enacted a law requiring shipowners to buy medical insurance for seamen. The law was signed by another notable framer: President George Washington. Congress followed this with a 1792 law requiring all able-bodied citizens to buy a firearm, and a 1798 law requiring seamen to buy hospital insurance for themselves. Today, there are a host of affirmative federal duties to buy things. For example, federal law requires corporations to hire independent auditors, and requires unions to buy insurance bonds in case their officers engage in fraud. The list goes on.

In all these cases (other than the firearms case), one could say the federal duty was imposed on persons who are already engaged in some commerce. But that is also true of everyone subject to the health-insurance mandate, because all of us buy or sell something. And yet in each case, Congress required people to enter into commerce in a different market than the one in which they voluntarily operate, which is precisely what the health-care law’s challengers claim makes this mandate “unprecedented.”

(Indeed, the challengers relied on a distinction between the markets for health care and health insurance that is far thinner than the one between the markets for, say, shipping and health insurance.)

It would have been much better had Verrilli raised these precedents—especially the ones involving the framers—when he was confronted with the claim that Congress has never required anyone to purchase anything before. But one can see why he might have chosen the more cautious route, since citing the precedents would have required him to squarely admit that “Yes, Virginia (and Florida), there is a government power to make us buy broccoli.”

Nevertheless, his failure to cite them left Justice Anthony Kennedy under the false premise that, as he put it, the mandate to buy insurance “changes the relationship of the federal government to the individual in the very fundamental way.” That’s just not true, and yet he relied on that premise to say that the government thus has “a heavy burden of justification” in defending the law.

Verrilli had a good alternative argument: although the uninsured might seem like they don’t participate in commerce, they are actually predictably active in the health-care market because we all get sick eventually, and when the uninsured end up needing medical care, others pay most of the cost. This argument is right on the economics, but it’s dangerous to hang your hat on it, since saying that failing to buy insurance equals commercial activity isn’t exactly intuitive for those without a Ph.D. in economics.

Had the government more squarely attacked the challengers' framing of the case months ago, it would have been much clearer to everyone why this case is not at all about a fundamental change in the relationship of individual to government. Fighting on this more favorable terrain, the government also could have better exploited the fact that the law’s opponents ultimately conceded that the mandate would be constitutional if Congress just called it a tax (or tax credit), or if it had been imposed by a state. It is hard to assert that “this mandate fundamentally alters the relationship of individual to government” and at the same time admit that “it would all be hunky dory if the mandate either used different language or was adopted by states.”

In the end, it may not matter. Justice Kennedy and Chief Justice John Roberts ultimately expressed some sympathy with the solicitor general’s position that the uninsured were inevitably active in the health-care market in a way that justified congressional action under the Commerce Clause. But Verrilli’s case would have been easier had the government read up on its Sun Tzu and not simply accepted the opponents’ framing of the case.