All you need is the following...
1 colony completely devastated by orbital bombardment.
1 passenger liner that is destroyed while touring the Blah-Blah Nebula fields...
1 Convoy of Tellarite merchants, besiged and destroyed, only one gets away.
In all cases they never signal, warn or threaten, they just act.
And they take nothing.
What everyone doesn't realize is that they have strayed into Romulan space and the Romulans want no visitors.
I know that owuld get me pretty scared. YMMV