So, what other technology do the Breen possess that might be used as a weapon?
There is no evidence that any Typhon Pact Member State possesses any other form of technology that constitutes a direct threat to Federation security the way slipstream technology did.
If Slipstream on it's own is a weapon then perhaps the Breen mission to steal it was justified?
From the Breen POV, it no doubt was. After all, the Breen view the Federation as a hostile, imperialistic force.
That's not relevant to whether or not the Federation has a moral and legal right to retaliate by making sure the Breen can't use slipstream technology.
In the world of international relations, there is no universal morality, only what the traffic can bear and whatever morality the participants bind themselves to.
And he believes they were.
I am not claiming that Alexander Siddig was accurately characterizing Dr. Crusher, just paraphrasing what he said. There are a lot of people here who seemed to me to be arguing that a doctor would never kill someone or participate in combat, and Bashir's actor disagreed with that idea.
So, let's get a cloaked ship from the Klingons and drive it into the Breen planet with their R&D facilities on it at .99c. That should put a serious dent into any research that they might have that would be used as a weapon. And there wouldn't be any sign of Federation involvement so nobody should have any problem with it.
The Federation clearly believes that mass murder of an entire planet is immoral and would therefore never do it. It also clearly believes that killing civilians who are building a system that poses a threat to Federation security is
not immoral.
I rather agree with both assertions.
If anything can be used as a weapon then anything is a target.
Except that anything
can't be used as a weapon, because there are plenty of technologies out there for which the Federation already has an effective defense.