Attacking with Fleets
You can attack a fleet belonging to another clan when it is within the highlighted area by right-clicking on it. If the target is further away than your fleet's movement extent will allow, it will take more than one turn to reach it and launch an attack. Once a fleet has reached its target the battle panel will be displayed, prompting you to fight, resolve the battle automatically or withdraw.
Raiding Trade Routes
You can attack an enemy clan's trade by simply selecting your fleet and right-clicking on their trade route. A proportion of their income from that route will be passed to your treasury whilst your fleet remains in place. An attack on a port, either by a fleet or by sabotage, will result in the disruption of trade and the cancellation of a trade agreement only if there are no other ports through which to re-route it.
Click here to view a video tutorial about this subject.
Punitive Forces
In Fall of the Samurai, the raiding of trade routes to the west by local pirates or western fortune hunters will eventually lead to punitive naval action by foreign navies present in the region. These punitive forces are not affiliated to either the Shogunate or the Emperor, they act simply to protect their sovereign interests.