。
She set off for Thailand.
Literal
She [topic-は] Thailand [to-に] [set off on-旅立った].
旅立つ ('to set off on a journey, to depart') is more atmospheric and weighty than 行く ('go') or 出発する ('depart'). The verb 立つ ('stand') here carries its older sense of 'rise to begin / set off,' and the compound captures the moment of departure rather than the journey or arrival. Often used for significant trips — long voyages, life-changing journeys, even as a euphemism for death (旅立ちました = 'passed away'). タイ国 (with 国 'country') is slightly more formal than the bare タイ, useful when the country name might be ambiguous with homophones (タイ also means 'sea bream'). に marks the destination.