What is ironic in lines 10 & 11 of the poem "War is Kind"?

Lines 10-11 in Stephen Crane's poem "War is Kind" are as follows:

"Great is the battle-god, great, and his kingdom—

A field where a thousand corpses lie."

Although the poem in its entirety is heavily ironic, those lines certainly carry their fair share of the irony! It's a lot of verbal irony here: the speaker of the poem is saying the opposite of what he truly means. By insisting throughout the poem and in the title that war is kind, and then pointing out how terribly cruel it is, the speaker sets up an expectation only to reverse it.

Lines 10 and 11 are very clearly ironic because they're saying that there's a "great" god of battle, with a kingdom, but the speaker probably means that if there is a god watching over the battles, then he's a terribly cruel one (one who takes lovers, sons, fathers, and souls and allows them to be viciously killed). Further, the speaker calls a field of dead bodies a "kingdom". He says it's a kingdom; he means it's a terribly gruesome battlefield or perhaps even a cemetery. Furthering the irony, "A field where a thousand corpses lie" does not seem like a kingdom that a "great god" would rule over, because kingdom in which all citizens are dead is not much of a kingdom at all. In other words, aside from determining a possible afterlife, this "great" god has no power of the dead that make up his "kingdom", implying he is not so great, after all. 

All of these instances of literally saying one thing and then figuratively meaning the opposite is what verbal irony is. You could point to other examples of verbal irony throughout the poem, too, such as "the virtue of slaughter" in line 20 (which means instead "the vice or sin of slaughter") or the "splendid shroud" from line 24 (which instead means the "sorrowful shroud," since it's the clothing that the dead son is wearing).