This is a nuclear bomb that has been placed in the lap of Donald Trump. This memo is a preemptive legal defense for mutiny against the presidential chain of command. To cut through the "command-ese": it essentially states that the US Army will hold to its oath to uphold the Constitution even if that means disobeying direct orders from the President.
This is unprecedented. The US military has never publicly released anything like this in its history. This is an organization that highly values its tradition of remaining a-political. The fact that they are taking a side should hammer home the current gravity of the situation.
We stand on the edge of a great precipice. One good push shall shove us into the dark, and where it will end, I do not know. The sins of our forefathers will be paid, and now we must choose how.