Switzerland’s national air carrier, the ailing Swiss International Airlines, said Tuesday it had declined an offer by Germany’s flagship carrier Lufthansa to buy it and would instead join the Oneworld alliance led by British Airways. A Lufthansa spokesman told news agency AFP the German company had wanted a merger but the tie-up failed to get off the ground because Swiss was unable to fulfil certain financial conditions. Swiss and BA have struck a deal aimed at helping the troubled Swiss airline with its financial problems after massive job cuts and heavy losses earlier this year. BA will provide Swiss with a guarantee of 50 million Swiss francs (€32.2 million) and in return will obtain rights to eight of the 14 valuable flight slots Swiss holds at London’s overcrowded Heathrow Airport.