That is incorrect. Spy's sole purpose is to hide and strike the marshal. Any other enemy will kill him. The spy is a one trick pony.

@Earthling Z3R0. He's right, though. And maybe your right, too. In the version of the game I have from the 90's, the Spy kills anything it attacks, except bombs, but loses to anything that attacks it. But you may also be right that now the rule is it only kills the Marshal, the other spy, and the flag. It would be interesting if they changed it to killing the highest ranked enemy unit so that if the Marshals double kill each other, then the Spy is still useful.

Maybe they did different rules in different countries. My 90's board game had the Spy as I described. He could only kill the Marshal (when striking first) and flags. (But the latter rarely happened because you never wanted to reveal your Spy until he striked the enemy Marshal.)

I've heard of it as a made up fan rule, but I cannot find anywhere on the internet showing it as a rule in an official release. I do collect, so I'd be interested to know what version (copyright year and country) I need to look for to acquire this version.

German Version. This is what I just found online to help you out. You might need a web site translator. Ranking shows that the Spy can only beat the Marshal.

https://www.spielregeln.de/stratego.html


Edit: There's a download link for the original rules book on that site too which also shows that the Spy can only beat the Marshal.

I wonder why they made different rules for different countries 😃

I agree