Can I declare War without losing Honor?

You can talk to your minister to send an envoy, who's gonna declare a political war on your enemy.

From the inofficial wiki

"By talking to this minister you can conduct many different affairs of state. Diplomats can be dispatched to the other factions, performing such actions as declaring war..."

Sadly, it would still cause problem with your honor rating and lord relations. It only prevents you from losing relationship with city/village/lord you wolud have to harras otherwise.

In order to declare war without consequence, you have to wait for event informing about border incident.