This year's incident began last week when Peruvian and Ecuadorean troops exchanged some fire and harsh words over a small stretch of unmarked frontier near the Cenepa river. The incident quickly escalated.
By Sunday, Quito had accused Lima of launching a "massive offensive". Dozens of people are reported to have been killed. Peru admitted yesterday that one of its army helicopters was downed by an Ecuadorean missile, killing five crewmen.
Both countries have accepted an invitation to attend a meeting of the Rio Protocol group today in Rio de Janeiro with representatives of the US, Brazil, Chile and Argentina.
The conflict has been over half a century in the making. There has been a ceaseless string of incidents across the lightly guarded 1,000-mile border since July 1941, when Peru occupied territory in Ecuador.
Sunday was the 53rd anniversary of the end of the war, but Ecuador has never become reconciled to the accord.
Work started on delineating the border between the two states six months after the Rio Treaty, signed on 29 January 1942. But, it was too good to last. In 1950, as work neared completion, the then Ecuadorean president, Galo Plaza, said that he consideredthe treaty inapplicable. Work stopped. Ten years later, Ecuador declared that the treaty was void.
It is not clear what triggered the latest fighting, which has been concentrated along a 50-mile-long strip of jungle known as the Cordillera del Condor. The area, site of the last war between the two, in 1981, has gold, uranium and possibly oil reserves.Ecuador has staked out its claim with military posts.
But analysts and diplomats believe that what lies behind the latest fighting may have less to do with national pride and mineral wealth than with the political needs of both leaders.
Peru's President, Alberto Fujimori, faces elections in a few months and may be trying to notch up a military triumph following recent successes against Maoist rebels.
On Saturday, Mr Fujimori said that Peru "firmly maintains its position to defend the boundary line'', insisting the fighting was inside Peru and that Ecuador was the aggressor.
He added: "We are making efforts, both governments, I understand, to keep this from escalating."
Peru, experts point out, has no real interest in sparking a full-scale war for territory that it already possesses.
President Sixto Duran-Ballen of Ecuador is ineligible for re-election and has little to lose, and perhaps everything to gain, by pressing his country's claim.
His tenure as President has been marked by accusations of failing to uphold Ecuador's interests in business deals with multi-national corporations. "He may now be trying to go down as the defender of Ecuador's interests,'' John Crabtree, of Oxford Analytica, said.
Ecuador yesterday announced a new tax to pay for the war, ordering state employees to contribute two days' pay.Reuse content