Let me educate you
[2:191] And fight in the cause of Allah against those who fight against you, but do not transgress. Surely, Allah loves not the transgressors.
[2:192] And kill them wherever you meet them and drive them out from where they have driven you out; for persecution is worse than killing. And fight them not in, and near, the Sacred Mosque until they fight you therein. But if they fight you, then fight them: such is the requital for the disbelievers.
[2:193] But if they desist, then surely Allah is Most Forgiving, Merciful.
[2:194] And fight them until there is no persecution, and religion is freely professed for Allah. But if they desist, then remember that no hostility is allowed except against the aggressors.
now read it all, not selectively
Then read the commentary if you STILL dont understand and cannot interpret
http://www.alislam.org/quran/tafseer/?page=251®ion=E1
Fitnah does not mean Disbelief in the Quran in the way you believe in this verse it means way different:
Fitna (or fitnah, pl. fitan; Arabic: فتنة , فتن‎: "temptation, trial; sedition, civil strife") is an Arabic word with extensive connotations of trial, affliction, or distress. A word freighted with important historical implications, it is also widely used in modern Arabic.
Fitna (word) - Wikipedia, the free encyclopedia
en.wikipedia.org/wiki/Fitna_(word)Wikipedia
The Shirk you mention applies to forcing Muslims to forego their faith by conflict