What effect did the panther-like clashing of the two armies have upon Henry?

This is an interesting question, but I'd venture to say that this particular comparison has a sort of effect of awe on Henry. Instead of taking great aims to avoid such violence, he now seems to have embraced it on account of its monumental significance.

I think he felt disoriented. The battle around him did not feel heroic or even straight forward. The place looked like an alien landscape to Henry. Check out this quote,

"He came to a fence and clambered over it. On the far side, the ground was littered with clothes and guns. A newspaper, folded up, lay in the dirt. A dead soldier was stretched with his face hidden in his arm. Farther off there was a group of four or five corpses keeping mournful company. A hot sun had blazed upon this spot."

There certainly is a surreal element to this mixed with panic and terror.