Self-defense against threats to national security and individual citizens is a core right and duty of all nation-states. No one seriously disagrees. And yet this week, the Mediterranean Sea will once again be the site of a dangerous attack on this basic right.

Activists from around the world, seeking to draw attention to the plight of Palestinians in the Gaza Strip, plan to launch a flotilla of ships from Greece to challenge Israel's high-seas Gaza blockade. Drawing attention to the Palestinian cause is legitimate, indeed important. And yet Israel's blockade is equally legitimate and important; it represents the essence of the nation-state's right to self-defense.

The flotilla organizers and participants have publicly emphasized that they will carry no arms and have no hostile intentions; they are on a humanitarian mission. Sound familiar? Similar promises were made in 2010 about a peace-loving flotilla sailing from Turkey. Those promises were quickly shattered.

The loss of life in last year's flotilla was tragic; it was also largely avoidable. After all, had flotilla organizers agreed to Israel's offer to land their cargo at the Israeli port of Ashdod, the goods would have been safely transferred to Gaza: Humanitarian mission accomplished.

But that wasn't the point. The point was a provocation of Israel, with a requisite photo-op. The needs of the civilian population in the Gaza Strip were a footnote. This week's flotilla, at least, is open about its motives. In a letter to President Obama, the American contingent acknowledged that its boat would carry "no goods of any kind for delivery in Gaza."

The organizers make the claim that Israel's blockade violates international law. That's a catchy accusation, but it's wrong in law and in fact.

According to international law, states can declare and impose a sea blockade as a means to prevent contraband from being smuggled into their territory. For the blockade to be lawful, the contraband must pose a danger to the nation-state. "Due notice" must be given that a blockade is in effect and that it will be defended. Israel's Gaza sea blockade meets these requirements.

As to the facts behind the blockade, in August 2005, Israel unilaterally disengaged from the Gaza Strip. All Jewish settlements there were abandoned, the Israel Defense Forces redeployed outside the Gaza Strip, and then Hamas assumed all civilian control for the Palestinian residents.

The result of disengagement was far different from what then-Prime Minister Ariel Sharon envisioned: From 2005 to 2008, more than 10,000 missiles were fired from the Gaza Strip into southern Israel, placing more than 700,000 Israelis in daily danger of attack. In December 2008, in response to the unabated missile attacks, Prime Minister Ehud Olmert ordered the IDF to destroy the missile-firing infrastructure and to attack Hamas members involved in the daily attacks that had significantly and consistently disrupted life for Israelis in southern Israel.

Since Hamas gained control in Gaza, Israel has carefully controlled the borders, and it established the sea blockade three miles off Gaza's shoreline. After all, some of those 10,000 missiles were surely smuggled in by sea. Israel's enemies characterize border control and the sea blockade as a nation aggressively preventing a civilian population from receiving much-needed assistance from the world; they call it a deliberate denial of Gazans' basic rights. And yet medicine, food and humanitarian aid can pass the border subject to security checks. To object to the blockade is to abandon the notion of national sovereignty and deny the right to national self-defense.

If the flotilla were truly a humanitarian mission, its organizers would act in concert with Israel to better the lives of the civilian population in Gaza. As it stands, should the flotilla set sail, Israel will enforce its legal sea blockade. To do anything less would abrogate its duty to its citizens and its right to self-defense under international law.