I iterate over the array and modify the contents of the array, but I do not get the expected results.

I iterate over the array and modify the contents of the array, but I do not get the expected results. What am I missing or is something wrong?

I have two div groups (one with an attacking class and another adversary) with three elements each. I am trying to select one element on each side, creating a border around it. Now I want to switch classes from attacking to enemy and vice versa. But when I use for loop, it somehow ignores some elements and changes only one or two div classes. Here is my code:

HTML:

<div id="army1">
    <div class="attacker">
        <img src="img/Man/Archer.jpg" />
        <div class="hp"></div>
    </div>
    <br><div class="attacker">
        <img src="img/Man/Knight.jpg" />
        <div class="hp"></div>
    </div>
    <br><div class="attacker">
        <img src="img/Man/Soldier.jpg" />
        <div class="hp"></div>
    </div>
    <br>            
</div>

<div id="army2">
    <div class="enemy">
        <img src="img/Orcs/Crossbowman.jpg" />
        <div class="hp"></div>
    </div>
    <br><div class="enemy">
        <img src="img/Orcs/Mine.jpg" />
        <div class="hp"></div>
    </div>
    <br><div class="enemy">
        <img src="img/Orcs/Pikeman.jpg" />
        <div class="hp"></div>
    </div>
    <br>            
</div>

And my javascript code:

var attacker = document.getElementsByClassName('attacker');
var enemy = document.getElementsByClassName('enemy');


var button = document.getElementById("fight");

// var class1 = document.getElementsByClassName("first")[0].getAttribute("class");
// class1 = class1.split(" ");

//choose attacker
for (var i = 0; i < attacker.length; i++) {
    attacker[i].onclick = function () {
        //select only one attacker and set its id to attackerid
        if (this.getAttribute('class') != 'attacker first') {
            resetAttackerClasses();
            this.setAttribute('class', 'attacker first');
        } else {
            resetAttackerClasses();
        }

    };
}

//choose enemy
for (var i = 0; i < enemy.length; i++) {
    enemy[i].onclick = function () {
        //select only one attacker and set its id to enemyid
        if (this.getAttribute('class') != 'enemy second') {
            resetEnemyClasses();
            this.setAttribute('class', 'enemy second');
        } else {
            resetEnemyClasses();
        }
    };
}

//fight
button.onclick = function() {
    //take off enemy health
    document.getElementsByClassName('enemy second')[0].children[1].style.width = '50px';

    resetAttackerClasses();
    resetEnemyClasses();

     for (var i = 0; i < attacker.length; i++) {
            attacker[i].setAttribute('class', 'enemy');
            enemy[i].setAttribute('class', 'attacker');
    };
};


function resetAttackerClasses() {
    for (var i = 0; i < attacker.length; i++) {
        attacker[i].setAttribute('class', 'attacker');
    };
}
function resetEnemyClasses() {
    for (var i = 0; i < attacker.length; i++) {
        enemy[i].setAttribute('class', 'enemy');
    };
}