cummings is mocking patriotic rhetoric. cummings mashes cliches that you would typically hear from politicians in speeches or traditionally, at fourth of july celebrations. this is a satire on a nationalistic or narrow-minded patriot who "did not stop to think" about their words or actions (war - slaughter). by mashing these cliches together, cummings illustrates the parrot-like or brain-washed way these speeches are strung together and that the quick fire string of patriotic cliches ultimately shows 'talking without thinking.' this is kind of prescient considering we now live in an era of media sound bites.
cummings is also mocking the narrow-minded idea that whoever challenges these concepts or the speaker is un-american. if the line "then shall the voice of liberty be mute?" is also sarcastic. the line follows the description of "happy heroes" "rushing to the slaughter" - patriots who are willing to die for their country. if they die, and they are the ones who fight for liberty, their voices, the voices of liberty will be mute (dead). but this is a satire of those who would rush to war simply in the name of patriotism; or more directly, the politicians who send soldiers to war. the voice of liberty may in fact be those who stop and question patriotic rhetoric and question the rush to war. so, maybe it is their voices which are muted by the shouting patriot with his mishmash of cliches.
Word inversion and disordered grammar were part of cummings' style. in this poem, they satirically resemble the cliched phrasing/structure of political speech.
This poem has echoes of Dulce et Decorum Est.