Jihad is part of the Islamic foundation to wage war and establish Sharia law in every corner of the earth.
If the country falls under the category of Dar al-Kufr (دار الكفر), translated as "House of Disbelief" or "Abode of Infidelity" (The "Dar" (دار) means "house" or "abode," while "Kufr" (كفر) means "disbelief" or "infidelity".)
As commanded in the Quran verses, Muslims must wage war against infidels until they submit or die. So Allah said in the Quran 9:111, Muslims must be prepared to kill and be killed for the Islamic faith.
In Ibn Kathir's tafsir provides further explanation.
