Plagiarism is the gravest of all academic crimes.
To retain the credibility of those who work in the field, history has very high standards for ensuring that all unoriginal work is properly acknowledged as such. Unless you use proper styles and standards of citation, plagiarism can easily occur unintentionally, in which case it is still a grievous crime. The easiest way to avoid accidental plagiarism is to scrupulously cite your sources, and to quote correctly. Make certain that paraphrased material is written in your own style and diction. Do not simply rearrange sentence patterns.