Problem with multiple loops in wordpress theme

The proper way to set up a secondary query, using your example code. Instead of this:

$posts = get_posts('numberposts=1&order=DESC&orderby=post_date&category='.get_cat_ID( 'سیاست' ));

Do this:

// Query args
$custom_cat_args = array(
    'posts_per_page' => 1,
    'order' => 'DESC',
    'orderby' => 'post_date',
    'cat' => get_cat_ID( 'سیاست' )
);
// Instantiate query
$custom_cat_query = new WP_Query( $custom_cat_args );
// Instantiate loop
if ( $custom_cat_query->have_posts() ) : while ( $custom_cat_query->have_posts() ) : $custom_cat_query->the_post();
    // Normal loop markup here
// Close loop
endwhile; endif;
// Restore $post global
wp_reset_postdata();

Using this method, your custom query will play nicely with your main loop query, and any other secondary queries.