Use Instantiated clone in the next function in same class

Hi, my question is probably really stupid but I can't seem to find a general rule for it or better, because sometimes it seem to work and sometimes not. Maybe someone can bring some enlightment to me :)

Here I add my variables in the beginning

     public GameObject attack;
     public GameObject defenseButton;
     GameObject attackClone;
     GameObject defenseButtonClone;

In the inspector I have dragged the prefabs for attack and defenseButton onto the slots in the script.

In the update function I instantiate the attack GameObjekt as affackClone (inside an if statement)

             attackClone = Instantiate(attack);
             attackClone.transform.SetParent(thisIsTheCanvas.transform, false);
             attackClone.tag = "AttackButton";

In the same script a little further down I have a function called AttackButtonClicked which get's called if the AttackButton gets clicked (On Click () of UI Button). Inside this script I want to destroy the attackClone. But in order to do that I have to define the variable attackClone again using FindwithTag.

     public void AttackButtonClicked () {
 
         attackClone = GameObject.FindWithTag ("AttackButton");
         Destroy(attackClone);
         ShowDefenseButton ();
         Debug.Log ("Button Destoryed" + attackClone);
     }

I just don't know why I have to assign the variable again? Shouldn't it work all over this script? If not why? Has it something to do with ActionButtonClicked being called by a UI Button? Maybe someone can explain this to me?

EDIT Here's teh fullcode after changing it from the suggestion from the first answer

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class SelectCountry : MonoBehaviour {
 
     public bool attackFromCountry = false;
     public bool attackThisCountry = false;
     public bool buttonIsThere = false;
 
     //GameObject attackingCountry;
     int unitsInAttackingCountry;
 
     //Canvas and Buttons
     GameObject thisIsTheCanvas;
     public Button attack;
     public GameObject defenseButton;
     public Button attackClone;
     GameObject defenseButtonClone;
 
     //Selected Countries to reset
     GameObject firstSelectedCountry;
     GameObject secondSelectedCountry;
     
     void Start (){
     
         thisIsTheCanvas = GameObject.Find("Canvas");
 
     }
 
     void Update() {
         if (Input.GetMouseButtonDown(0)){
             Debug.Log ("LeftClick");
             RaycastHit2D hit = Physics2D.Raycast(Camera.main.ScreenToWorldPoint(Input.mousePosition), Vector2.zero);
         if (hit.collider != null) {
                 // Select Country to attack from
                 if (attackFromCountry == false && hit.collider.tag == "Untagged"){
 
                     //attackingCountry = hit.collider.GetComponent<CountryBaseClass>();
                     Debug.Log ("wossers");
                     unitsInAttackingCountry = hit.collider.GetComponent<CountryBaseClass>().unitsCountry;
                     if (unitsInAttackingCountry >= 2){
 
                     hit.collider.tag = "isActiveAttacker";
                     attackFromCountry = true;
                     Debug.Log ("wow");
                     }
                 }
 
                 // Select Country you want to attack and make it possible to deselect country
                 else {
 
                     if (attackThisCountry == false && hit.collider.tag == "Untagged" && attackFromCountry == true){
                         hit.collider.tag = "isActiveDefender";
                         attackThisCountry = true;
                         Debug.Log ("super");
                     }
                     else {
                         if (attackFromCountry == true && hit.collider.tag == "isActiveAttacker" && attackThisCountry == false){
                             hit.collider.tag = "Untagged";
                             attackFromCountry = false;
                         }
                         if (attackThisCountry == true && hit.collider.tag == "isActiveDefender"){
                             hit.collider.tag = "Untagged";
                             attackThisCountry = false;    
                         }
                     }
 
                 }
 
         }
         }
 
 
         // Show Attack Button if a two countrys are selected
         if (attackFromCountry == true && attackThisCountry == true && buttonIsThere == false) {
             attackClone = Instantiate(attack);
             attackClone.transform.SetParent(thisIsTheCanvas.transform, false);
             attackClone.tag = "AttackButton";
             buttonIsThere = true;
              
         }
         // Destroy attack Button if country is deselected
         if (buttonIsThere == true && attackThisCountry == false) {
         
             Destroy(attackClone);
             Debug.Log ("Button Destoryed" + attackClone);
             buttonIsThere = false;
            }
     }
 
 
     // Check if attacked button was klicked and then disable it and show defend button
     public void AttackButtonClicked () {
 
         //attackClone = GameObject.FindWithTag ("AttackButton");
         //Destroy(attackClone);
         attackClone.enabled = false;
         ShowDefenseButton ();
         Debug.Log ("Button Destoryed" + attackClone);
     }
     
     public void ShowDefenseButton () {
 
         defenseButtonClone = Instantiate(defenseButton);
         defenseButtonClone.transform.SetParent(thisIsTheCanvas.transform, false);
         Debug.Log ("Show Defense");
 
     }
 
     public void DefenseButtonClicked () {
     
         defenseButtonClone = GameObject.FindWithTag ("DefenseButton");
         Destroy(defenseButtonClone);
         //reset
         firstSelectedCountry = GameObject.FindWithTag ("isActiveAttacker");
         firstSelectedCountry.tag = "Untagged";
         secondSelectedCountry = GameObject.FindWithTag ("isActiveDefender");
         secondSelectedCountry.tag = "Untagged";
 
     }
 }