He used the excuse of protecting his own nation.
Treaty of Versailles banned Germany from using much armed force,and limit it's army to 100000 men,along with the rules of not allowing navy, air force and limited the amount of heavy weapon.
When Hitler took over Germany,he used the excuse of defending from other nations to start producing weapons, navy and air force again as it was greatly disarmed and unable to protect itself if invasion occur.
Hope it helps!
