Ironclad ship battle

[Eva Elisabeth]

I read (can't remember where) that - although inconclusive - the ironclad ship battle on March 9, 1862, was the beginning of the modern war aera. The British, who previously had regarded their armada (also including ironclads) as the superior one, were shocked and alerted and especially considered the northern "Monitor" to be unbeatable. WHY? I'd also like to know if it was, like the Merrimac, an originally wooden ship rebuilt with iron all around or if it was entirely made of steel?