Complex sentence: The battle--centred around the Gemencheh Bridge--proved costly for the Japanese, who suffered up to 600 casualties but the bridge itself--which had been demolished during the fighting--was repaired within six hours.
Split into simpler sentences: The battle--centred around the Gemencheh Bridge--proved costly for the Japanese, who suffered up to 600 casualties. However, the bridge itself (which had been demolished during the fighting) was repaired within six hours.