Why when Allah strikes he uses a simple tools but it causes a harsh results?

he killed nemrod by a gnat that intered his head so he decided to exchange his head by a golden head which didnt work at the end after he cut his real head,and he saved the prophet mohammad by a spider web when he was hiding in the cave,and now his skinny jihadis will cos the u.s troops in iraq to rebel,why he uses this policy?is it to prove to his enemies that it doesnt take much to ruin them?y people dont learn from the lessons?y they continuing to rebel on Allah?