Yii2 Delete Alert box not working - php

Question

I generated a model,controller and a view by Gii,And I enabled the actionColumn in index.php.
I have delete button to delete the records.The buttons are working good but,It doesn't shows me the Alert box to confirm my action.
I am attaching my code here.
Can anyone tell what I am missing?
Index.php
<?php
use yii\helpers\Html;
use yii\grid\GridView;
$this->title = 'Cities';
$this->params['breadcrumbs'][] = '/ '.$this->title;
?>
<div class="cities-index">
<h2><?= Html::encode($this->title) ?>
<?php //echo $this->render('_search', ['model' => $searchModel]); ?>
<?= Html::a('Create Cities', ['create'], ['class' => 'btn btn-success','style'=>'float:right']) ?>
</h2>
<?= GridView::widget([
'dataProvider' => $dataProvider,
//'filterModel' => $searchModel,
'columns' => [
['class' => 'yii\grid\SerialColumn'],
// 'country_id',
[
'attribute'=>'country_id',
'value'=>'country.country_name',
],
'city_name',
'city_tax',
[ 'class' => 'yii\grid\ActionColumn',
'header'=>'Action',
'template'=>'{view}{update}{delete}',
'buttons' => [
'view' => function ($url, $model) {
return Html::a('<span class="btn btn-xs btn-success icon-search bigger-120" style="margin-left:5px;"></span>', $url, [
'title' => Yii::t('app', 'View'),
]);
},
'update' => function ($url, $model) {
return Html::a('<span class="btn btn-xs btn-info icon-pencil blue bigger-120" style="margin-left: 5px;"></span>', $url, [
'title' => Yii::t('app', 'Update'),'<span class="glyphicon glyphicon-search"></span>'
]);
},
'delete' => function ($url, $model) {
return Html::a('<span class="btn btn-xs btn-danger icon-trash bigger-120" style="margin-left: 5px;" ></span>',
$url, [
'title' => Yii::t('app', 'Remove'),'<span class="glyphicon glyphicon-search"></span>',
'data-confirm' => 'Are You sure',
'data-method' => 'post'
]);
},
],
],
],
]); ?>
</div>