Korea ? U.S. and South Korean warships and helicopters practiced anti-submarine maneuvers off the Korean peninsula today, readying defenses against the kind of weapon that allegedly sank a South Korean navy vessel earlier this year.

The destruction of the Cheonan in March, which has been blamed on a North Korean torpedo, killed 46 sailors in the worst military disaster for the South since the 1950-53 Korean War.

The four-day “Invincible Spirit” exercises involving 20 ships, 200 aircraft and about 8,000 U.S. and South Korean sailors are being held in response to the sinking, bringing threats of retaliation from North Korea, which denies responsibility for the attack.