The almost 20,000 British soldiers were killed on the first day of the Battle of the Somme due to several reasons:
1. **Inadequate planning:** The British military leaders did not plan the attack properly. They underestimated the strength of the German defenses, leading to a poorly executed offensive.
2. **Artillery bombardment:** The artillery bombardment preceding the attack failed to destroy the barbed wire and other defenses of the German forces, leaving the British soldiers vulnerable to heavy machine gun fire when they went over the top.
3. **Communication issues:** There were communication problems between the soldiers and their commanding officers, causing confusion and disorganization on the battlefield.
4. **Lack of effective tactics:** The tactics used by the British Army were outdated and did not adapt to the modern warfare techniques employed by the Germans, resulting in high casualties.
5. **Limited intelligence:** The British forces had limited intelligence about the German defenses and positions, which contributed to the failure of the initial attack.
Overall, a combination of poor planning, ineffective artillery bombardment, communication issues, outdated tactics, and limited intelligence led to the high number of British casualties on the first day of the Battle of the Somme.
