The US Fifth Army was activated on this date, 5 Jan 1943.
Part One of Fifth Army History can be seen here:
Fifth Army history. Part I, from activation to the fall of Naples.
This .pdf file, ANNEX NUMBER ONE Letters and Orders of Activation , conyains the activation letters & troop assignments:
http___cgsc.cdmhost.com_cgi_bin_showfile.pdf
Is the "540th Engineer Shore Regiment" our 540th? If so, I've never heard it called that.
Maj Todd O. USMC, Retired
Grandson of LTC John O'Brien
Yes, it is. They acted as SHORE REGIMENT, one of their several designations.
The 540th have gone under these various names:
540th Engineer Shore Regiment
540th Engineer Combat Regiment
540th Engineer Combat Group
Larry, thanks for posting this GREAT STUFF! I sure appreciate it. 
Marion J Chard
Proud Daughter of Walter 'Monday' Poniedzialek
540th Combat Engineer - H&S Company
I've added both of these documents to the main site. Many thanks!
Marion J Chard
Proud Daughter of Walter 'Monday' Poniedzialek
540th Combat Engineer - H&S Company