Brazil, Colombia and Peru have signed an agreement to join together in the fight against arms and drug smuggling across their shared borders.
Colombia hopes the move will deal a blow to rebels
Under the accord, the countries will share information about smugglers and co-ordinate military operations
It was signed by the three countries' defence ministers on a boat in the Amazon, where the frontiers meet.
Correspondents say the deal will be welcomed by Colombia in particular, in its fight against left-wing rebels.
It will, they say, send a signal that Colombia's neighbours will not tolerate incursions.
The pact marks the first time in recent history that the three armed forces have launched a joint action.
"It is very difficult to have a physical presence along the whole frontier," said Peruvian Defence Minister Roberto Chiabra.
"With this, one could put forces in a blocking position if there are operations in one of the other countries."
Brazilian Defence Minister Jose Viegas said it was important to "unite our forces against our enemies".
The move is seen by Colombia as a significant step in the fight against the Revolutionary Armed Forces of Colombia - or Farc - rebels, who tax the drug trade to finance their war.
"This is intended to complicate the lives of the Farc," said a Colombian defence ministry official.
Colombian authorities estimate that 20% of Colombia's drugs are transported along the Amazon to Brazil and other markets.