Everybody can register for his nation.
13 players by nation with 13 differentes rosters.

Then:
Step 1. We randomly sort the first match.
Step 2. Losers of the game are eliminated from the competition. Draws and wins go to the next round.
Step 3. Counting the survivor.
Step 4. The team with less remaining players gets decides pairing of the 2nd round.
Every player calls an opponent. Players not challenged wait for the 3rd round.

Step 5. Repeat from Step 2. Until one team is defeated.

A player who waited in a round MUST play the next one, so he must be chosen by the opposing team, providing they have enough players.

Last rule.
If, after 10 assaults, no team has been defeated, the match is a draw.