The leagues system represents player's personal rating. Ranks and rewards are given to players depending on their position in rating leaderboards.
You can be matched against a player from another league while participating in Clan Wars or in one of the special events which are introduced into the game from time to time and offer unique rewards. In these cases the leagues do not influence the selection of opponents for you, since Clan Wars are aimed at gaining dominance over the whole server leaderboard and events are mostly represented by PvE missions. Clan Wars matchmaking is based on the overall power of all members of the team that takes part in battle.
The event competitions are not influenced by the league system, because event leaderboards are common for all players on the server and dynamic gameplay is required to provide maximum amount of players with a chance to compete within the given period of time. Nevertheless, all the opponents are matched so that every player could have a chance to win if the battle disposition is favourable to him or her.