Biological weapons are too indiscrimate and slow-acting to serve as a deterrent; (1) If Country A uses bio weapons to retaliate against Country B, the weapons may kill not just people in Country B, but in Countries C, D, E, etc. as well, then spread back to Country A, and (2) bio weapons take weeks or even months to really start racking up body counts.

A capacity to strike enemy leadership, say by assassination.

The type of non-nuclear deterrent would depend upon who you wanted to deter. Biological weapons are dangerous and unpredictable, but the threat of their use against an enemy's major city could be an effective deterrent only if the threat were credible. In order for it to be credible you would have to first prove that you had biological weapons, and then you would have to prove that you have some method of delivering them. This could be conterproductive because it allows the enemy to form countermeasures. If you plan on delivering them by aircraft they could beef up their air defenses, and if you plan to deliver them by missile you run the risk that your enemy may perform a pre-emptive strike to take those assets out before they can be mobilized. The same could be said for chemical weapons. The shear horror of their effects on the population may be enough to cow a rival nation and keep them from taking any belligerent action. I seriously doubt chemical or biological weapons would be effective deterrents against a European nation. Such a threat would probably only provoke a military attack. I hate to say this, but I feel that terrorism would probably be an effective weapon against a Western European nation. Spain demonstrated this on 3/11, and I'm afraid other nations in Western Europe would react similarly. Let's use Iran as an example. Let's say that that after the United States leaves Iraq, Iran invades and then threatens Kuwait and Saudi Arabia. The international community is outraged and threatens military action against Iran. While the countries of Europe begin mobilizing their militaries, suicide bombers starting blowing themselves up in schools, shopping malls, and churches in Berlin, Paris, Brussels, Rome, Prague, and Madrid (among others). The government of Iran threatens that more attacks will follow unless the Western nations stand down and stop mobilizing against Iran. This strategy would (probably) not work against the United States or Britain, but I bet that it would have a crippling effect in many other European countries. Terrorism is a low tech and immoral deterrent, but I'm afraid it would probably work on some countries.

A carrier battle group represents a sizable deterrant. They cost more than missiles, but if you are limited by treaty they might offer some diplomatic leverage.

perfectgeneral: I would have said the same thing. The point being that any weapon with trained personel and intact command structure is a deterrent. Few countries want to go to war, as it is so costly. On top of that I would say a good civil defence system has the advantage of saying: If you hit us and we as much as notice it ...... Doesn't work with the irrational minds of terrorism, though ....

James Dunnigan made the same point about aircraft carriers in his book "A Quick and Dirty Guide to War." He discussed how useful a carrier task force might have served the British during their Falklands campaign in 1982. The only fighter aircraft the British could deploy from their ships were Harriers. Harriers made good CAS aircraft, but were never meant to serve as fighters. A carrier equipped with F-4s or F-14s could have effectively dealt with the Argentinian threat posed by Mirages and Entenards. The Brits couldn't keep the supersonic jets away, and they lost men and ships as a result. Dunnigan then points out that had Britain possessed some modern carriers in 1982, the Argies might not have invaded the Falklands. It is interesting to note that Argentina had an aircraft carrier during that campaign, but the threat posed by British submarines intimidated the Argentinians into keeping it in a port in Argentina during the war. That carrier was meant to deal with regional threats, not against a major naval power. If the Brits sunk their carrier, they were screwed. Perhaps an effective submarine fleet that can deploy anywhere in the world in a short period of time is also an effective deterrent.

doh, caps lock... well, the Sovs in Lebanon...sent kidnapers' relatives home in pieces, right? So maybe a good 'Stan Yellow Pages.

Tahat is exactly the problem with terrorist: They are undeterrable. Everybody else has something they want to keep or have, and that is usually a basis for negotiation.

Well in that case, what they want to keep may be family members in one piece. You know, they talk all hard-core, but I wonder if many underneath aren't sniveling rats who can indeed be broken - assuming Miranda and Geneva can be induced to take a little walk somewhere for a spell.

"What do you think would be the best strategic deterrent assuming the country in question cannot have nuclear weapons"

Geography. (ok this has a lot to do with luck of the draw, but at least it doesn't cost much)

Plenty of nations are deterred simply because they can't easily get to an adversary or getting there puts them in an adverse position.

However, barring that.. poor geographic decisions by your adversary.   The biggest example I can think of currently is the location of Seoul, South Korea.  Ok, so North Korea maybe has put together some nuclear weapons..  However, they have had a significant detterent of having  large quantities of artillery in range of their enemies capital city, with a population of close to 11million.   Who needs a nuclear weapon then?  That capability is probably one of the largest factors that allowed them to develop nuclear weapons in the face of potentialy hostile and overwhelming international intervention.