The issue of we define "strength" seems to be the first question to be answered. If "strength" is the ability to use force, then goodness probably is correlated with weakness, since the good are more reticent to resort to the use of force.
If "strength" is the ability to resist the use of force by others, then it is more complicated. The "good" need not eschew the use of force in self-defense, though you then get subsidiary questions of which uses of force are defensive. For example: was a pre-emptive war in Iraq "defense"? Obviously for those of us who don't see that Saddam posed us any realistic threat, it is hard to say that war was defensive. Many do not view the war Afghanistan as defensive either, though I do, so the answer to the question will obviously turn out to be subjective.
On the use of torture, it is hard to imagine something more evil than the intentional infliction of anguish on someone who is completely at your mercy. There you have to answer the question of whether the efficacy of torture makes it worth the evil plus worth the various secondary effects (such as the increased likelihood of torture being used on your people and the inability to credibly argue against its use when other people do it). In light of the secondary effects, though, it's hard to argue that eschewing the use of torture is "weakness" since the use of it seems to inflict some costs on us that are just as dire. Even that assumes that the torture will lead to actionable intelligence, which in reality it might not.
In addition to all that, you have to realize that the "game" of international relations is a long term one. The use of "evil" methods might seem expedient and there might be situations where no other options seem to be available, but the use of these methods has a lasting impact. People remember them and their hatreds can last well into the next conflict.
Nearly 150 years later, and I still have to listen to southerners accuse the United States of "aggression" in the Civil War—notwithstanding that it was done to preserve the Union that many of those southerners paradoxically claim to love—because it is so hard to get perspective on the use force, even in a good cause. At the same time, I still cringe at what they did at Andersonville and their blatant attempts to steal U.S. property.
Jews still remember the years of their slavery in Egypt and that happened thousands of years ago. Blacks remember it so strongly that many want reparations for what their ancestors suffered.
Evil acts, even when used in self defense, can cast a long shadow that haunts us into the future. So, even when the use of force may be morally justified, it seems to me more prone to leading to adverse unintended consequences down the road. As such, the strong preference for peaceful resolution of disputes, even when frustration leads us to see no alternative to the use of force, has some merit as it is more likely to lead to a lasting peace than ending a conflict by force would be.
Would Gandhi have been "stronger" if he had used suicide bombing against the Brits? Would the Civil Rights Movement have been stronger if they'd shot Bull Conner dead or tortured the locals in Philadelphia, Mississippi to find out who killed James Chaney, Andres Goodman, and Michael Schwerner. I am sure just murdering Bull Conner had to have occurred to somebody, where they weak for having refrained? In both cases there were also people who thought that a non-violent movement against the oppression they faced was dumb and that the oppressors would triumph.
Because the long term consequences of the use of force can be so dire, we need the more pacifistic elements to help rein in our natural inclination to indulge in it. It's good to be forced to confront the question of whether the methods we're using are necessary or merely expedient, and if the latter whether they are worth it.
