To which military operation does the expression in the Portuguese language “a cobra vai fumar” (“the snake will smoke”) refer?

Yet Brazil's quasi-neutrality would come to a sudden end in the summer of 1942. Earlier that year, Kriegsmarine u-boats sank several Brazilian merchant vessels off the East Coast of the United States. Then, in June, Hitler decided Brazil's pro-U.S. stance constituted an act of war, and launched a "submarine blitz" in retaliation. In August alone, a single long-range submarine, U-507, sank six Brazilian ships over just two days ...

For over a year, the Brazilian Army slowly gathered sufficient numbers of men to fill the ranks of the Brazilian Expeditionary Force (Força Expedicionária Brasileira, or FEB). The new force, it was agreed with the United States, would be led by General João Batista Mascarenhas de Morais - at 61 years old, the oldest Allied divisional commander in Europe - and organized to American military standards. But it took so long to recruit, organize, train, and negotiate its deployment overseas that Brazilians began to joke that the FEB would go to fight "a cobra vai fumar" - literally, "when snakes smoke," or in English, "when pigs fly." With its antique equipment, untested reserve officers, and half-learned and out-of-date French military doctrine, the FEB was distinctly unappealing to American officers ... On Sunday, July 16, 1944, the first detachment of 5,000 Brazilian troops reached Naples. Brazilian leaders had envisioned sending an army of 100,000 to participate in Operation Torch. But the North African campaign ended before they could get there, and the force was scaled down to a single large infantry division, the 1st Expeditionary Division (1st DIE), and an air detachment.