There are three things that non-proliferation folks talk about; mass to low earth orbit, guidance, and multiple payload deployment.
If you can put something into a pre-chosen orbit you've got the launch and navigation down (after all an ICBM in in 'orbit' that just happens to be highly elliptical and intersects the planet rather than goes around it.) But there is always the mass problem, conventional bombs, combined with relative lack of orbital precision, means that even a 2,000 lb bomb which is a 'big' iron bomb if you can't accurately get it within a mile or two of its target it won't be very effective. A nuclear weapon clearly can 'miss' by a couple of miles and still be very effective, but they are really really heavy unless you know what you are doing. The first bombs built by the US weighed in about 5 tons, but they did damage equivalent to more than 20,000 tons of explosive (a 'gain' of 4000). State of the art weapons have much higher yields. But if you're new at the game you have to have a rocket that can lift 5 - 6 tons before you are a 'threat' to the rest of the world.
"I suppose I always figured the main thing restricting proliferation is engineering the payload and not the delivery system."
Well if you can build a bomb, but the only way to hit someone with it is to fly it in on a huge transport plane, or drive in with it on a truck, it is both easy to defend against and you have plenty of time to figure out if you need to defend against it. If on the other hand you can launch it into space and have it fall out of the sky some where in 45 minutes to an hour, that requires a different strategy on the part of folks you might seek to attack.
If you can put something into a pre-chosen orbit you've got the launch and navigation down (after all an ICBM in in 'orbit' that just happens to be highly elliptical and intersects the planet rather than goes around it.) But there is always the mass problem, conventional bombs, combined with relative lack of orbital precision, means that even a 2,000 lb bomb which is a 'big' iron bomb if you can't accurately get it within a mile or two of its target it won't be very effective. A nuclear weapon clearly can 'miss' by a couple of miles and still be very effective, but they are really really heavy unless you know what you are doing. The first bombs built by the US weighed in about 5 tons, but they did damage equivalent to more than 20,000 tons of explosive (a 'gain' of 4000). State of the art weapons have much higher yields. But if you're new at the game you have to have a rocket that can lift 5 - 6 tons before you are a 'threat' to the rest of the world.
"I suppose I always figured the main thing restricting proliferation is engineering the payload and not the delivery system."
Well if you can build a bomb, but the only way to hit someone with it is to fly it in on a huge transport plane, or drive in with it on a truck, it is both easy to defend against and you have plenty of time to figure out if you need to defend against it. If on the other hand you can launch it into space and have it fall out of the sky some where in 45 minutes to an hour, that requires a different strategy on the part of folks you might seek to attack.