Bradford believed that the Separatists, as they called themselves, were carrying out God's will by leaving England (and the Netherlands, where they had lived for around ten years) and establishing a new Zion in America.  He saw parallels between biblical stories and the trials they faced in Massachusetts and wanted to leave an account to inspire succeeding generations to maintain their beliefs and practices.  

In the journals that informed his narrative, Bradford gave very little credit to the people of the Massachusetts Bay Colony for their courage, perseverance, or intelligence.  Any positive outcome (i.e., anyone being spared an illness or untimely death) was credited to God, such as the miraculous survival of John Howland, who fell overboard from the Mayflower, and the serious illness Bradford himself suffered in the winter of 1620.

William Bradford explains his purposes for writing the History of Plymouth Plantation in Chapter 6 of the text. He begins by saying that he recorded the information so that the descendants of the Pilgrims will appreciate the hardships their forefathers faced as they searched for a place to worship their religion freely.

I have been the larger in these things, and so shall crave leave in some like passages following, (though in other things I shall labour to be more contract) that their children may see with what difficulties their fathers wrestled in going through these things in their first beginnings...

In addition to appreciating the hardships of their ancestors, Bradford desired the future inhabitants of Plymouth to know that they did not persevere through these hardships by their own strength alone; rather, God had provided the strength and help they needed to endure every struggle. Thus he described numerous instances in which he believed God had helped them.

and how God brought them along notwithstanding all their weaknesses and infirmities.

Finally, Bradford hoped his detailed record of life in Plymouth could be an example for future people facing similar predicaments.

As also that some use may be made hereof in after times by others in such like weighty employments; and herewith I will end this chapter.

Bradford explains his reasons for writing his detailed history of Plymouth Plantation at the end of chapter 6 of this book. He states:

I have been ye larger in these things, and so shall crave leave in some passages following, (though in other things I shall labour to be more contract,) that their children may see with what difficulties their fathers wrestled in going through these things in their first beginings, and how God brought them, along notwithstanding all their weakness and infirmities. As also that some use may be made hereof in after times by others in such like weighty employments; and herewith I will end this chapter.

He states three reasons for writing this history: first, he fervently wishes ("craves") that descendants of the first settlers know how difficult it was to establish the colony. He doesn't want the memory of this epic struggle to be lost or minimized. Second, he perceives the book as a testimony to God's providence in protecting the pilgrims despite their weaknesses. Finally, he hopes that if future generations face such severe challenges ("weighty employments") that his account can act as a guidebook or help for them.

These three goals—and especially the first two— impact how Bradford tells the story. First, he doesn't want to gloss over the difficulties or turn the tale into a heroic narrative. He is at pains to fully describe the sufferings and the failures of the first settlers, so that the extent of what they accomplished against high odds is fully appreciated. Second, he is at pains to demonstrate that settling in the New World is the fulfillment of God's plan for them and that God is an intimate part of how events unfolded. For example, in chapter 10, Bradford notes that in the earliest days:

here is to be noted a spetiall providence of God, and a great mercie to this poore people, that hear they gott seed to plant them corne ye next year, or els they might have starved

The story Bradford wants to tell is that the earliest Pilgrims suffered, but God meant for them to settle and prosper in this new land.

To answer this question we need to carefully consider the purpose of William Bradford's History of Plymouth Plantation. This book is generally thought of as a history of life in early colonial New England. Because it is the only work of it's kind to address this time and place, it is usually studied in school as an example of colonial literature and an important historical document. However, we need to keep in mind that students, especially high school students, are typically exposed to only a small portion of the book.

The oft-used high school textbook Elements of Literature—Fifth Course has only excerpts from Chapters 9 and 11 of the book. Entirely omitted is the story of the Puritans in Holland and England prior to their journey to America. This part of the book, which covers about 14 years of Puritan history, details their difficulties in finding a place to worship. By the time the finally reach the New England shore in September, 1620, they have already faced many hardships and deprivations.

Bradford's purpose is not so much to illustrate an important aspect of America's origin and development, but to show how the Puritans are working in accordance with God's will to establish a Godly settlement in which they can live according to their own beliefs.

We see this religious emphasis in Bradford's interpretation of events. When relating the fate of a particularly unlikable non-Puritan sailor, Bradford writes:

But it pleased God before they came half seas over, to smite this young man with a grievous disease, of which he died in a desperate manner.

Bradford attributes the death of the sailor to God's vengeance. In fact, just about everything is attributed to God's will in one way or another. It is entirely natural for Bradford and the Puritans to look at life this way, so we see quite a few similar instances throughout History of Plymouth Plantation. Bradford wants to show the that the Plymouth colony has successfully fulfilled God's plan.  

This book was written by William Bradford, who governor of the Plymouth Colony for over 30 years.  The Plymouth Colony is the colony that was founded by the famous Pilgrims, who came to America in 1620.  In the book, Bradford tells the story of the founding of the colony and its subsequent history.

Bradford starts his story with the Pilgrims' desire to leave Holland and come to America so they could live by the rules of their Puritan faith.  He discusses the hardships of the voyage on the Mayflower and of the early "starving time" after the colony was established.  He then continues the narrative through the first Thanksgiving and the rest of the colonies early history.  The account ends in 1647.

As the title suggests, History of Plymouth Plantation was intended by Bradford to be an account of the founding of Plymouth. Indeed, it is a valuable primary source--in some cases, the only primary source--for many of the events that have now become very famous, including the landing at Plymouth, the Mayflower Compact, and the Thanksgiving feast. Moreover, Bradford discusses the reasons the Pilgrims chose to depart England, the tribulations they faced on the way to North America, their complex interactions with Native Americans, and above all, their struggles to establish a permanent colony after landing. Perhaps the dominant theme in Bradford's account, and certainly the most evident to a modern reader, is the extent to which the author attributes the successes of the Pilgrims (and their failures as well) as part of God's plan. This, Bradford makes it clear, is the intended message of the work, and how he hopes the Pilgrims will be remembered:

What could now sustain them but the Spirit of God and His grace? May not and ought not the children of these fathers rightly say: "Our fathers were Englishmen which came over this great ocean, and were ready to perish in this wilderness; but they cried unto the Lord, and He heard their voice and looked on their adversity,"