Your supposed to be a viking, there are no rules

As John wrote, a lot depends on the kingdom, how new it is, and who thinks they should be in charge, also, some players have very thin skins and take affront at the least thing, even when there is no provocation.

First, did you know that the tile was occupied when you sent your march?

In order for there to be a battle on a tile , at least one of the parties has to have no sheld. 

If you dropped your shield to attack, its a different matter to not having a shield sending to a tile an arriving ther after  someone else. either because they were nearer,move faster,or purposefully sped up to beat you there.

Though it does happen, some players can take offense if you go to a lair after they have killed the invader/despatched the ghost.