the sinking of the bismark
You battle Kyogre for your chance to capture it in Pokemon Alpha Sapphire after you capture the groudon.
A "Forlorn Hope"
You will battle Groudon for your chance to capture it in Pokemon Omega Ruby after gaining more power.
One thing that happened by chance is that the winds changed, so that William's boats could cross the Channell to begin the battle.
Battle of Antietam
chance
chance
Win a battle on union soil.
it has a %2 chance of appearing
the climate, the ocean, the cold winds from the ocean, and the chance of hurricanes.
the episode is called "A Triple Fighting Chance"
Has a 1 out of 24,576 chance of being a wild Pokemon you catch or battle, which is 1/3 the chance of getting a shiny Pokemon.