Given the following sentence in the American Declaration of Independence:
But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security.
I understand the sentence as a substantiation for the making of a new US government, however what does the particular highlighted phrase mean?