For anyone looking for a quick summary, the problem is orbital inclination (i.e. how tilted the orbit is relative to the equator). Columbia was in a 39 degree inclination, and for reasons related to physics (their launch site is at a high latitude) and politics (China lies to the east) the Russians can't launch into inclinations less than 51 degrees. It's possible to change inclinations after launch, but requires a metric buttload (technical term) of fuel.

This is all leaving aside the fact that the Russians (or, indeed, anybody) don't just have rockets sitting around ready to go at a moment's notice.

Are you talking about the thread with Mvandenbergh and tanzam75? Based on:

> If you launch from Baikonur directly into a 46° orbit, you will drop spent rocket stages on China. That is why the Russians launch into a 51° orbit, even though their launch site is at 46°.

> With a Proton, you have enough delta-v to launch into 51° and then make a change of plane to 39°.

It could have made the orbit just fine it just wouldn't be able to boost the shuttle into the ISS's plane. It probably could have brought them supplies if China agreed to allow the spent boosters to fall.