Question

What is "shadow war," and how is it different from previous forms of warfare?

Answer 1

Well, I believe what you mean by "shadow war" is a "proxy war", which is when a larger power (often 2) oversees a smaller war between countries, where it influences it but does little fighting.

Explanation:

Good examples of this can be seen in the Vietnam War at the beginning, and throughout the Cold War.
The USSR were supplying the North Vietnamese with weapons to fight again the Americans/South Vietnamese, such as anti-aircraft missiles and tanks.

The Americans originally over saw the South Vietnamese, who were somewhat controlled by the US during the conflict, and the army was supplied and given training by the US.

Overall the US failed and the North Vietnamese (and Communism) won.