This latter phrasing is certainly the way that any sensible president—the person who decides whether to commit more of the nation's blood and treasure to a war—would have to read that sentence. His military commander on the ground is telling him that if he doesn't send more troops, he'll lose—but if he does send more troops, he still might lose.
McChrystal's point is that it's not simply "resources," not just U.S. and NATO troops, that will settle the war. It's also whether the Afghan government earns the trust of its people—whether the Afghan president and his entourage of ministers, governors, and warlords are willing—or are willing to be lured—to clean up their act, end their corrupt practices, and truly serve their people.
When Obama says he needs to review the strategy before he decides on troop levels, he almost certainly means that he needs to assess whether a counterinsurgency strategy makes sense if the Afghan government—the entity that our troops would be propping up and aligning themselves with—is viewed by a wide swath of its own people as illegitimate.
Obama committed himself to a new strategy for Afghanistan this past March. He is now wavering, not so much because many congressional Democrats and a majority of the American people have turned against the war. (Congress would almost certainly vote in favor of appropriations, just as it did in the bleakest days of the Iraq war, if just to "support the troops," and a successful battle or two might well turn public opinion.)
Rather, the big new thing that's happened since March—in fact, since McChrystal and his staff prepared their memo over the summer—is the Afghan presidential election, which, it's turned out, was marred by fraud on a monumental scale, nearly all of it on behalf of the incumbent, Hamid Karzai. Even so, Karzai seems to have barely tipped the 50 percent required to avoid a second-round runoff. If he is declared the winner and offers nothing to the runner-up, Abdullah Abdullah, popular trust in his government will slide still further—and the prospects for a successful counterinsurgency campaign will slide with it.
In other words, Obama is right to hold off on making such a huge decision. He's right to wait and see how the Afghan election plays out and how Karzai behaves in its aftermath. The McChrystal memo emphasizes that the only reason for sending more troops is to implement the new strategy. "Without a new strategy," he writes, "the mission should not be resourced"—that is, no more troops should be sent. The same is true if the new strategy has scant hope of succeeding.