Nearly 3 1/2 decades after leaving the Soviet Union, the Baltic countries of Estonia, Latvia and Lithuania this weekend will flip a switch to end electricity-grid connections to neighboring Russia and Belarus — and turn to their European Union allies.

The severing of electricity ties to oil- and gas-rich Russia is steeped in geopolitical and symbolic significance. Work toward it sped up after Russian President Vladimir Putin ordered his troops to invade Ukraine three years ago, battering Moscow's EU relations.