Lines 5-8 Summary

Get out the microscope, because we’re going through this poem line-by-line.

If we must die, O let us nobly die,

Line 5 starts a new section in the poem that is going to build on the first four lines.
The speaker repeats the beginning of line 1, but changes the last half of the line to suggest that simply not dying like hogs isn't good enough. Instead, the speaker wants his group to die in a noble way.
This idea of a "noble" death brings to mind images of heroic deaths in battle. It's still a common image today that we see in lots of books and movies.
For a poem written in 1919, it's a little weird to use "O." But it does add some flare by making the end of the line very dramatic.

So that our precious blood may not be shed
In vain;

For the speaker, dying nobly means that his death won't be meaningless or "in vain."
"[P]recious blood" sounds a bit like biblical language, and brings to mind images of Jesus Christ being sacrificed.
The whole idea of a "noble" death gets tied up with Christianity, and McKay is using this to fit his own message. We have to wait and see what that is.

then even the monsters we defy
Shall be constrained to honor us though dead!

The attackers, which were first referred to as dogs, are now monsters.
But even thought the attackers are terrifying, the speaker and his allies are going to resist.
"[D]efy" means to rebel against a power.
The speaker says that he and the people he's speaking to are going to die so bravely that even the monstrous enemy is going to be forced to respect them.
That is, after they're all dead.