You must have no troops of your own. You can only cause damage equal to a portion of your own army size (troops in this case), or a portion of your target's army size, whichever is smaller. This limits the amount of damage a small(er) country can do to a bigger one, particularly since a smaller country can always drop land to get a higher SPAL.

For example, for stealing tech, you can only steal 0.3% of your target's tech or 0.4% of your tech, whichever is the smaller value.