Educational Decree Number Twenty-two

'So now we know how we ended up with Umbridge! Fudge passed this "Educational Decree" and forced her on us!
-- Hermione Granger (OP15)

"I am here to teach you using a Ministry-approved method that does not include inviting students to give their opinions on matters about which they understand very little. Your previous teachers in this subject may have allowed you more licence, but as none of them - with the possible exception of Professor Quirrell, who did at least appear to have restricted himself to age-appropriate subjects - would have passed a Ministry inspection --"
-- Dolores Umbridge (OP15)

States that should the Headmaster of Hogwarts be unable to fill a position, the Ministry will find an “appropriate” candidate. (OP15)

Passed on 30th August 1995, just before Harry's fifth year (OP15)
Umbridge was hardly a good candidate for the job of Defence Against the Dark Arts professor, since her daily command of "wands away" made certain the students would get no real-life experience using defensive spells in class (OP15, OP16).
Minister for Magic Cornelius Fudge also wanted to reign-in Dumbledore so he wouldn't form a student-army that might work against the Ministry, and he knew that Umbridge would be the perfect Ministry spy (OP14, OP15).
Hermione and Harry decided to fight back by forming the very thing Fudge feared the most - Dumbledore's Army, training themselves in Defence Against the Dark Arts since Umbridge would not allow them to use their wands in class (OP16).