When we talk about the form of a poem we normally are referring to such aspects as how the poem is structured and positioned on the page. If you have a look at the poem carefully, it is actually written in iambic pentameters with some variation. It is divided into four irregular stanzas of, respectively, 8 lines, 6 lines, 2 lines and 12 lines. The biggest variation in the meter is the last line, which only has 6 syllables, therefore emphasising the ending and the harsh, bitter criticism in the Latin quote that gives this excellent poem its title.
When we consider rhyme scheme, it is actually far more consistent. Consider how the rhyme scheme goes abab cdcd efef ghgh ijij klkl mnmn. The regularity of this rhyme scheme serves to emphasise the disturbing mood of the poem as we are forced to see the disparity between the harsh reality of war and romantic idealism, which is often portrayed through just such a regular rhyme scheme.
It is mostly in Iampic pentameter. It strays from this in the last lines of the last stanza.