MySQL group by like statement?
Two things are needed to make it run faster:

Get suitable index

Without confirmed = 1, you need

INDEX(country, date_created)


With it, you need

INDEX(country, confirmed, date_created)


Avoid hiding the date in a function

LIKE '2015-10%' cannot use an index on date_created, nor can MONTH(date_created), so let's reformulate the query:

WHERE date_created &gt;= '2015-10-01'
  AND date_created  &lt; '2015-10-01' + INTERVAL 1 MONTH
  AND confirmed = 1   -- add this if you want to limit to confirmed rows


(This assumes that date_created is a DATETIME or TIMESTAMP datatype.)

All Months

This query can still use the two indexes I suggest, however, it won't necessarily use the date_created part.

SELECT LEFT(date_created, 7) AS YYYY_MM,
       COUNT(*) AS ct
    FROM tbl
    WHERE country = 'DE'
      AND confirmed = 1   -- if you want this
    GROUP BY YYYY_MM;


Note about country_code

A common mistake in defining the schema (please provide SHOW CREATE TABLE) is to define it as CHAR(2) and default to utf8.  This leads to using 6 bytes, when 2 would suffice.  Change to CHAR(2) CHARACTER SET ascii.