Complex sentence: In the United States, the term "libertarianism" generally refers to neo-classical liberalism, and this meaning has diffused beyond the USA's borders via political parties and think tanks since the 1970s, causing libertarianism to be increasingly viewed worldwide as a free-market ideology.
Split into simpler sentences: In the United States, the term "libertarianism" is often used as a synonym for "neo-classical liberalism". This meaning has diffused beyond the USA's borders via political parties and think tanks, and libertarianism is increasingly viewed worldwide as a free-market ideology.