'great wall of steel', I suppose is a takeoff of 'the iron curtain.'

The potentiality that concerns me is that someone will try sneaking in a just a small tactical nuclear blast, just to see how or if the world will react.

How WOULD the USA react if Russia, for instance, set off a small one in some mostly deserted area? They could do that with the accompanying argument that they are gathering data.

Would the US consider that provocative enough to respond in kind?