international trade

International trade is exchange of capital, goods, and services across international borders（分界线） or territories(领土，版图）.
If we do not have international trade , we will become more poor. For example we need imports materials , foods, products etc, such as banana, tea,
Silk, gas. Also like wine,Spacecraft, if we do not have material to make , our production will change to inefficiency, or lower efficiency.international trade good for every countries which are join it.
But free trade also bring some drawback to those countries , for example clothes seller afraid of the clothes ,which are imports, are cheaper , the demand of their clothes will decrease. Also when the customer change their mind to buy Japanese car instead of the domestic car , they producer and seller should worry about the demand of their goods.
Risks in international trade
Economic risks
§ Risk of insolvency(无力偿还的） of the buyer,
§ Risk of protracted default （拖欠债务）- the failure of the buyer to pay the amount due within six months after the due date
§ Risk of non-acceptance（拒绝承兑）
§ Surrendering（放弃） economic sovereignty（最高统治权）
§ Risk of exchange rate
§ Susceptibility（易受影响或损害的状态） to changing standards & regulations within other countries

Political risks
§ Risk of cancellation（取消） or non-renewal(不能重建）l of export or import licenses
§ War risks
§ Risk of expropriation（征收） or confiscation（充公） of the importer's company
§ Risk of the imposition（强加） of an import ban after the shipment（ 从海路、陆路或空运的)一批货物）of the goods
§ Transfer risk - imposition of exchange controls by the importer's country or foreign currency shortages
§ Surrendering political sovereignty
§ Influence of political parties in importer's company
§ Relations with other countries

So some countries not real curry out completely free trade , because they know , there are not only have profit , but also have cost. Thus the government of those countries adopt some way to limited it.
1) tariff , which can limited imports.
2) Quota, limited the quantity of imports
3) manage exchange rate.
4) Imports permit.
5) Refuses , for example drug
Etc
All of those way are help limited trade.