The first Europeans to land in Tonga were the Dutch in 1616, then the Dutch again arrived in 1643 when Abel Tasman arrived. Despite these early arrivals, they made no effort to settle the islands or colonize them. It wasn't until the 1770s when James Cook of the United Kingdom arrived that the culture and people truly began to change.
These stops by James Cook led to a vast influx of Christian missionaries and by the mid-1800s almost all of the people had converted to some form of Christianity. These missionaries gave the people new reason to live as their morals and priorities changed with the new religions. It also forced the people to abandon their past religion as their culture and lifestyle began to change dramatically.
In the political realm from the 1770s to the mid-1800s, the internal conflicts had continued as they had prior to European arrival. Since no true foreign government had taken control of the islands during this time, and the local people were divided, the only outside influence came from the missionaries who were slowly, but successfully converting the people. The end of the warring and the successful conversion of the people coincided as both essentially ended in 1845 when the local chief Taufa'ahau was baptized and proclaimed King George Tupou I.
King George Tupou I made significant changes to the country and its people as he shifted his government to reflect that of the United Kingdom, while following Christian ideologies. He essentially mandated and legalized (or illegalized) various aspects of the past that put the people's lifestyle and their culture more in alignment with that of Christian beliefs. He also created a constitutional monarchy modeled after the British government.
King George Tupou I's changes vastly altered the culture as they became more and more Christian in name, in practice, and in belief. His rule also ended the violence of civil war and established a fairly stable country allied with the United Kingdom. This was done in part through religion as the people were unified under this new religion as opposed to remaining divided based upon political or economic issues.
Despite King George Tupou I's successes, this government didn't last long and in 1900 the new king's power was in jeopardy, leading to a Treaty of Friendship between Tonga and the United Kingdom. This treaty essentially guaranteed the king's power in exchange for a loss of political and economic power to the United Kingdom. This again stabilized the country and the government, but kept power in the hands of the local kings and chiefs as the British slowly gained more influence from these chiefs or via these chiefs as they became allies.
Due to stability and, perhaps more importantly, the relationship Tonga held with the United Kingdom from the mid-1800s, the culture became fairly westernized. However, the economy has never been extraordinarily strong so purchasing power has remained weak. Despite this, the access to new technologies, advanced communication, and transportation have made Tonga, in many ways, a reflection of the west. Traditional housing and food still exists, but this time period truly westernized the country.
After years of stability, British rule ended in 1970 as Tonga became an independent country. Little changed with this political transition as the British had allowed the Tongan kings to rule the islands for years. In fact, the British never directly controlled Tonga and today this long-term voluntary relationship between the two countries continues.
Despite the smooth transition and stability over the years, Tonga has recently faced economic hardships. Depending on current farming and trade, the country's economy is heavily susceptible to changes in demand as well as the local weather. This inconsistency has led to a slight alteration in the power structure of Tonga. In 2008 the king, King George Tupou V, called for a more democratic process and handed many parts of the government, in particular the day to day details, to the elected government. This move has stabilized the government, although the economy is still inconsistent.