The West is nervous of the contents of the Russian convoy, now headed for Ukraine supposedly to bring humanitarian aid to the civilians cut off by the fighting between separatists and Kiev forces. The fear is of a Trojan horse carrying arms and ammunition within the food and other supplies.
First of all the people hiding in cellars and woods as fighting rages around them, without food and fuel and water, represent a humanitarian emergency, regardless of whose side anybody is on. They deserve and should get aid. The Russians are best placed to supply this and their convoy should be allowed through. There is a risk it is carrying ammunition or even weapons and military personnel, but the West can afford that risk; if Russia were found to be smuggling weapons in white lorries it would be hugely damaging to Putin’s credibility.
On the other hand if the convoy is stopped, it would provide a pretext for Russian military intervention to open a humanitarian corridor, very much on the lines of the operation now being conducted in the Middle East, by the US and Britain and France. Is that what the West wants? Because that is what it will get if it does not proceed with care and realism. Remember there is now an endless list of foreign policy initiatives which have achieved the opposite of the intention.