The history has proven repeatedly that wars could lead to extremely terrible disasters. (中上之作)
或是
The history keeps proving that wars could lead to extremely terrible disasters. (中等之作)。
另外要注意一點,「導致」這個動詞片語雖然有很多同義字,但不代表每個都適用於本句,若使用像是 contribute to 這類有正面含意的片語,會導致扣分。
至於第二題,「避免衝突、確保世界和平....」這兩個詞應該用 Ving + Ving 的形式構句:Avoiding conflictsandensuring world peace should be the goals that all human beings are after/pursing. (我自己寫的參考答案)