One of Several Reasons Why You Shouldn't Attack Your Allies!

[matteus the inbred]

if you can afford to fight (and technically lose) some small battles try sending in repeated small armies of 5-6 Welsh Bandits and specifically shooting up the huscarles before withdrawing...tricky to pull off, but i did it against the Mercians once and wiped out two units of huscarles and 400 other guys besides...you might want to take some junk and cavalry to protect the bandits while you do it though, sometimes the AI sits there and doesn't attack, sometimes it comes after you...once you've shot them all up you can invade properly with standard stuff.
oh, forgot to add...hire mercs...javelins (Irish) and crossbows (Pictish?) and target huscarles with those as well.
as for your king, if you build loads of cheap stuff he'll probably get the Builder and Magnificent Builder virtues which will increase happiness and then loyalty.
but yeah, basically don't backstab! strangely, people get resentful when you do that...