Armistice Day Challenge

JavaScript (ES6), 57 bytes

n=>" ".repeat(n).replace(/./g,"||  ||  $'//$`  ||  ||\n")

05AB1E, 24 bytes

<ðׄ//J¹FD"  ||"2׊««,À

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Explanation

                          # implicit input n
<ð×                       # push n-1 spaces
   „//J                   # join with "//"
       ¹F                 # input times do:
         D                # duplicate top of stack
          "  ||"2×        # push "  ||  ||"
                  Â       # push "||  ||  "
                   Š      # move the top of the stack down 2 places on the stack
                    ««    # concatenate the top 3 elements of the stack
                      ,   # print with newline
                       À  # rotate top of stack left

Previous 26 byte version

F"||  "2שð¹N-<ׄ//ðN×®RJ,

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Perl, 45 bytes

-9 bytes thanks to @Gabriel Benamy

47 bytes of code + 1 byte for -n flag.

say$@="||  "x2,$"x$_,"//",$"x++$.,$@while$_--

Run with -nE flags :

perl -nE 'say$@="||  "x2,$"x$_,"//",$"x++$.,$@while$_--' <<< 5