As Russian gas cutoffs upend European energy security, the continent is struggling to cope with what experts say is one of its worst-ever energy crises—and it could still get much worse. For months, European leaders have been haunted by the prospect of losing Russia’s natural gas supply, which accounts for some 40 percent of European imports and has been a crucial energy lifeline for the continent. That nightmare is now becoming a painful reality as Moscow slashes its flows in retaliation for Europe’s support for Ukraine, dramatically increasing energy prices and forcing many countries to resort to emergency plans, and...