Two warships from the Philippines and Indonesia will set sail today from Davao City in the southern Philippines to jointly patrol the Celebes Sea. They are scheduled to reach Manado, in Indonesia, next Wednesday.
The two countries have been conducting these yearly coordinated patrols since 1986. But this latest one comes under the growing shadow of Islamist militancy, and as bloody clashes between Philippine troops and Muslim militants for control of the southern Marawi City drag on for a seventh week.
That this is the third joint sea patrol in this part of the world in less than a month illustrates the scale of the problem that everyone now recognises and is working hard to tackle. But it is an immense task.
Intelligence reports suggest that up to 200,000 boat trips a year between the Philippines and Indonesia are unchecked.
The Philippines has issued arrest warrants for about 400 suspected terrorists, all with ties to militants who seized large parts of Marawi on May 23 to turn it into a "province" of the ultra-radical Islamic State in Iraq and Syria (ISIS).
Many militants are believed to have already left the war-torn southern Mindanao Island for safe havens elsewhere in the Philippines, and in Malaysia and Indonesia. Those in Malaysia and Indonesia, meanwhile, may be planning to head to Mindanao to continue fighting there and plotting attacks across South-east Asia, long after government troops have retaken Marawi.
This is the kind of threat that this week's drills are meant to address.
Last week, the Philippines was joined by the United States in patrolling its southern waters, where the small but brutal Abu Sayyaf group, an ISIS vanguard in the region, had been abducting people.
But South-east Asia's littoral borders are porous. It will take more than a few ships sailing together to make them safe for everyone.