specifically after the crisis in 1948 and again in 1871—-

I’m assuming you meant 1848 and 1871 … everybody else’s answer dated back to the Revolution, and that isn’t in the scope of the question. I’m not sure about 1848, but I’ll give you the 1871 example. It boils down to the Franco-Prussian war and the beatdown given to France by Bismarck and the Prussians. French command and leadership failed, Napoleon III was captured in the field at Sedan, and the French fell apart. The French were increasingly insecure in relation to the new German state, and once war broke out it was pretty much a done deal. The French were forced to sign a peace treaty, losing Alsace-Lorraine to the Germans. After that point, with Napoleon III already abdicated, the French began anew with a republican government.