war, fighting, battle
戰 centers on armed conflict: war, fighting, and battle. It captures the idea of organized violence between opposing forces.
戰 combines 單 (originally a hunting weapon or net) with 戈 (halberd), together expressing warfare or fighting. The modern Japanese simplified form is 戦.
The left side 單 suggests a weapon or tool, and the right side 戈 is a halberd. Picture two armed sides clashing: that is war and battle.
For セン, imagine a thousand (千, せん) soldiers marching into battle: 千 -> セン, and the whole scene is war.