Complex sentence: Iberia is allied with American Airlines, Qantas, Avianca, British Airways, PLUNA of Uruguay and Grupo TACA, and on 1 September 1999, the company joined the Oneworld alliance.
Split into simpler sentences: Iberia is allied with American Airlines, Qantas, Avianca, British Airways and Gruop TACA. On 1 September 1999, the company joined the Oneworld alliance.