Not German but the issues here are generic. If they could do that it almost by definition wouldn't be a coalition. Nuclear is apparently a red line for the Gree

Not German but the issues here are generic. If they could do that it almost by definition wouldn't be a coalition. Nuclear is apparently a red line for the Greens. If they were to be kicked out of government then that government would collapse. Forming new governments can take a very long time in European PR systems - half a year isn't unheard of. The choice here is really one of either: 1. Convince the Greens to change. If they do they'll probably be destroyed at the next election, because they have relatively few well known positions and they already abandoned some of them. If they abandon their opposition to AKW then they will be left with nothing. 2. Collapse the government, meaning no decision making ability at all. The latter may potentially open up the possibility of new alliances, but they would be between ideologically opposed parties and therefore highly unstable. Lesser of two evils, right?