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If you're the Germans it's a bit more complicated. At the beginning of the war they faced the nightmare scenario of being pulled into a two-front war with France on one side and Russia on the other, something which previous German leaders had sought to avoid. Their initial plan, the Schlieffen Plan was to knock France out of the war before Russia could fully mobilize and threaten Germany, mobilization in those days took weeks or months. Then they'd rush their forces from France to meet the Russians.

If you're the Germans it's a bit more complicated. At the beginning of the war they faced the nightmare scenario of being pulled into a two-front war with France on one side and Russia on the other, something which previous German leaders had sought to avoid. Their initial plan, the Schlieffen Plan, was to knock France out of the war before Russia could fully mobilize and threaten Germany; mobilization in those days took weeks or months. Then they'd rush their forces from France to meet the Russians.

If you're France and their allies, it's because you want the Germans out of France. The Germans aren't just going to just go away.

And we've never seen its like since. A single, continuous, fully manned line is too costly and too brittle for the pace of modern warfare. The Great War taught the armies of the world how to assault a line, and it also taught defenders how to be flexible rather than be rigid lest they be outflanked and bypassed. For example, Finland's Mannerheim Line was really a series of mutually supporting bunkers, dugouts, and trenches. If an attacker overruns any one position, they will take fire from multiple supporting positions. If multiple positions are overrun, the defenders fall back to another line.

If you're France and their allies, it's because you want the Germans out of France. The Germans aren't just going to just go away.

In any other situation you'd bypass a strong enemy position and cut off their supply. This avoids fighting the enemy where they're strong and forces them to come out to attack you. But the continuous line of trenches in the Great War made this tactical mobility impossible.

Russia was eventually defeated in 1918. It did result in the Germans refocusing on France, though leaving far too many troops to garrison their conquered Russian territory. But by then it was too late. Germany and her allies had been bled white. US troops were arriving to bolster the Allies. When the German Spring Offensive hit the western front in 1918 with new tactics they finally got their breakthrough, but they no longer had the manpower to exploit it.

Russia was eventually defeated in 1918. It did result in the Germans refocusing on France, though leaving far too many troops to garrison their conquered Russian territory. But by then it was too late. Germany and her allies had been bled white. US troops were arriving to bolster the Allies. When the German Spring Offensive hit the western front in 1918 they did so with new "stormtrooper" tactics. The Germans used fire-and-movement to break the stalemate and finally got their breakthrough. But they no longer had the manpower to exploit it.