On the first line the author contrast the previous utopic images with the 'darkling plain' to emphasize the fact that the world is not what it seems to be , by using 'here', a present tense, he states that it is currently happening.
The 'struggle' refers to the one of humans, the struggle these have when searching for the purpose/ meaning of their life. Their 'flight' indicates the rough journey humans pass whilst searching for this purpose.
The words swift and clash have connotations with the motions of the sea, the sea symbolizes faith in this poem, so it can be said that this specific words represent the coming and retrieving of faith, faith was once abundant in people and know as time passes it seems to fade.
On the last line Arnold may have compared the crash of the waves to human war. Also, the ignorant armies could be to an extent, groups of faithless people that share the characteristic of having no spiritual beliefs/
Sir im reaaaaaally sleepy so this 'analysis' is probably not making any sense. But I did it! I suggested the idea of having them gathered around the computer and they left me, so greaat po sir. Byeeee