Note: Section 18 is different in later editions of "Song of Myself." In the later editions the section is devoted entirely to images of war, which were likely influenced by Whitman's experiences in the Civil War. We're still talking about the 1855 edition here, but don't worry, we'll get back on the same page in Section 19.
Whitman continues to expand on the idea that everyone shares in the truly essential things in life: songs, water, laws, and so on. It's hard to tell if he is being literal or metaphorical and comparing his thoughts to these essential items.
On the subject of war, he says that his song is for the defeated as much as the victorious. Those who failed in battle are no less deserving of praise than those who succeeded. He doesn't divide the world into "winners" and "losers."