Yes, it can do that. Refer to page 20 of the P40 rulebook, under restrictions on placement:
“New Sea units can enter play even in a hostile sea zone. No combat occurs because the conduct combat phase is over.”
I think in Poland, the calvary was attacking German inf, and then tanks showed up.
You would be wrong to think that. Polish calvary and German tanks never clashed. The Polish horsemen ran like hell when they saw tanks because they weren’t insane.
You just finished me sentence.I did not mean to imply the cavalry decided to fight the tanks.