Way back in the 70s, when the BETA first made landfall in central Europe, NATO and the Warsaw pact forces tried to take out the hive with conventional weapons (tanks, Cobras, Hinds, what have you). They were unsuccessful in containing the hive, and BETA succeeded in making a breakout into central Europe. In short order, most of Europe was devoured, with the survivors retreating to England.
At present day, the UK houses the surviving remnant of the European Union, and the European Front is essentially the Battle of Britain mixed with BETA. BETA make repeated attempts to invade England, and have so far been repulsed - another light novel sidestory, Shwarzesmarken, follows a group of German TSF pilots who're holding the line.
Several attempts have been made to invade back into Europe, but have been unsuccessful.
There's another front in Siberia, though this is technically not quite the European front, where the Soviets are holding the line against BETA by the skin of their teeth, with everything but Kamchatka having fallen, which is why they're leasing Alaska from the US.
...heh. First time I'm the expert, instead of Myssa. Nostalgic, isn't it?