Raiding can often be far more effective than a formal and prolonged war.

I remember once as the French, I held Aquitaine and Toulouse and my enemy Spain held Navarre and Aragon, so there was a constant border war going on on both sides of the Pyrennees. We both had huge armies defending these important terroritories and stalemate had set in, a decisive defeat could result in total defeat in the war. So both myself and Spain attacked and counter-attacked each other never really offering a final confrontation.

Tired of this expensive and dangerous state of affairs I gathered a small army of 900 men and landed them in Portugal, the soft underbelly of the Spaniards, the province was seized and put to the sword I totally destroyed everything in the province. Then immediately invaded Corduba and did the same again leaving the province bare. All the time the Spaniards on the frontier where unable to effectively help the provincial forces for fear of invasion from Aquitaine or Toulouse. I did the same in Granada, but was eventually cornered on my way back to the coast in Leon, where I narrowly won a battle over a large provincial army leaving 750 dead to 400 lost, Leon was then sacked.

I then swiftly got my troops to the fleet and returned them to Brittany. This small, fast and sharp raid had totally devastated 4 provinces in Spain, totally destroyed much of the infrastructure of the Spaniards' best territories.

The war continued for another 10 years, but Spain was finding it harder and harder to replace its losses let alone afford them, eventually I broke through the frontier and crushed their armies one by one before knocking Spain into a fatal civil war.

The raid of 900 men had achieved what close to 9000 could not achieve for either side in 25 years of constant warfare, this is an extreme example of what can come from a raid, most wont be that successful. But with decent timing and a well picked force you can cause significant carnage, even if it failed it wouldnt have significantly harmed me anyway.