The cloak of Shi`ism is so wide and comprises various sub sects and doctrines. Some of them are extremists who unjustly curse the Companions of the Prophet (peace and blessings be upon him) or may even claim Ali (may Allah be pleased with him) to be a prophet. Others are less radical and more moderate. Thus, the position of a Sunni Muslim cannot be same with regard to all the Shiites. The less differences in major basics of belief are, the more openness and cooperation could be achieved.
Concerning a Sunni defending his or her Shiite country, Dr. Hatem Al-Haj, Professor of Fiqh said:
A Muslim mainly gives all allegiance and deliberate love (vs. the natural love one feels for family and people who are kind to him) to the ultimate source, the Creator of all things, i.e. his Lord, Allah. Then, based on people’s distance from and proximity to Him, he should decide how much of his allegiance and deliberate love he will allocate to each individual and group.
Being good to one’s country is a wider manifestation of being good to one’s neighbors. However, that shall never, for a true Muslim, supersede his allegiance to his Lord and His cause of good and justice.
Cursing the Sahabah (the Prophet’s Companions) is an enormity, and there is avery respected position within Ahlul-Sunnah that cursing Abu Bakr and Omar and accusing them of disbelief amounts to major kufr.
For the Sunni Muslims in Iran , it would be expected to support their country against aggression from a non-Muslim country. After all, it is a matter of measuring the distance, and a Muslim, is closer to you than others, even if he is an innovator.
To work for the cause of justice and the God-sanctioned human rights in any land is working for the cause of Allah, and will improve the conditions of all, including the Muslims.