Have you been confused by all the fooforaw around Boris Johnson and Brexit? I just read a quite good step-by-step recounting of how we got to where we are now, at the Arc Digital website. I recommend it to you, with one minor caveat.
My caveat is this, I don’t believe the article’s author does justice to the Northern-Ireland’s-border-with-the-Republic-of-Ireland issue. The Good Friday agreement (mostly) settled the long-running violence between NI Protestants who favor continued association with the United Kingdom, aka Britain, and NI Catholics who much prefer all of the island of Ireland to be ruled from Dublin by the Republic.
That agreement relied on both nations (UK and RI) being EU members with no border controls between them. This meant people could freely move between them, trade across the boundary with no controls, and most of the time ignore the governmental realities.
Leaving the EU, depending on how a border is reinstituted, can be said to effectively break the Good Friday agreement. This may lead to renewed violence.