Comrade has already put the reason why marine fandom trembling after seeing Shanks feats but what about this :
Previous fleet admiral's reaction on Shanks arrival and later his statement and instant peace agreement :
First Sengoku was irritated and mad about Shanks interference.
Later same Sengoku agreed to make peace instantly after first Shanks challenged Marine and BBP and later after BBP instant retreat, Sengoku agreeing with Shanks terms saying
IT'S YOU, so I'll allow it.
Marines were totally in Bloodlusted mode after killing WB and weren't stopping even after BBP started tantrum. But seeing Shanks challenging everyone, this is the reaction of not only marines but also pirates and that is after Shanks saying this:
Even Blackbeard saying similar thing. Same BB who was confidently fighting marines. :
Now bonus for this thread, the current fleet admiral and the man who has main role in Defeating WB :
Bloodlusted Akainu who for maintaining justice was on killing spree and was even willing to kill his own. But after seeing Shanks stopping his punch and feeling his CoC (considering Coby fainted while foaming as well), staying frozen on his spot and giving these growling reactions :
Now a question for marine fandom, marines were already Bloodlusted, while even after BBPs retreat, Marines were available there in full force and even no harm to there core. Shanks was there alone with main crew and no allies. But why they couldn't accept Shanks challenge and even went to agree on making peace while there leader said he's accepting terms only because IT'S YOU (Shanks).
Marine force along with warlords are there to put balance against 4 emperors, so just one yonko either pushed them to extreme diff (i seriously don't believe this) to make them very tired and in pretty bad shape or is Shanks and his crew just too much for them to deal with ?
Only Mihawk and BBP left after Shanks arrival while rest were all there.
Or for them, success would only be there if they can find another Squardo in RHPs who could first severely injure Shanks before admirals can start there work !?