在日本偷袭珍珠港事件以前，日本大量从美国尽快必要的战争物资，尤其是石油。在珍珠港事件之前，美国开始执行对日本的物资禁运。很多人因此认为，如果日本不去作这个死，集中全力去解决中国问题，那么抗战胜负，未可知也。
但大多数普通人不知道的是，事实上到 1941 年日本的外汇储备已经枯竭。也就是说，即便美国不去制裁日本，日本也不会有钱去找美国购买至关重要的战略物资（如石油，橡胶等）。在这种情况下，日本铤而走险，抢占法国、荷兰在东南亚的殖民地以获取战略物资（例如法属印度支那的橡胶和婆罗洲的油田，还包括粮食、铁、有色金属等）就成了无可奈何的必然选择。日本的外汇枯竭，事实上是日本侵略中国导致国力日渐枯竭的后果。这篇文章主要就这一问题进行考证。
1 外汇枯竭
知乎上一个高赞回答提到了这个问题。
其实原因很明显：日本侵华亏惨了老本，经济濒临崩溃，购买石油、粮食等战略物资的能力已近于枯竭，到1941年，按美国驻日大使的说法，日本可用于支付的外汇只相当于2万德国马克。（约瑟夫。C。格鲁《使日十年》）
在中文网络上，搜索关于1941年日本外汇储备的情况也几乎都能追溯到这个引用。不过这个引用细究一下有点奇怪。作者约瑟夫·C·格鲁是美国驻日大使，日本政府的财政状况，他何以清楚呢？毕竟当年不是现在，日本政府、军部应该不会定期公开发布全面的财政报告。
《使日十年》(Ten Years in Japan)，是 1932 年至 1942 年间担任美国驻日大使约瑟夫-C-格鲁(Joseph Clark Grew)的日记及公私文件摘录。该书的英文原版可以在这个链接查看。在该链接中，第399页，章节 Frozen credits bring japan close to bankruptcy写道:
October 9, 1941
According to information received by a member of my staff from what is regarded as a very reliable source, the amount of foreign exchange available to the Japanese Government now is approximately 20,000 reichsmarks, and under the circumstances the Japanese Government will be unable to avoid defaulting on contracts calling for foreign exchange on maturity. The freezing regulations which were put into effect by the United States, Great Britain, and the Netherlands East Indies have completely cut off any exchange transactions in the currencies of those countries, and in addition have greatly reduced transactions in the currencies of South American countries. According to this information, his own contracts involve foreign exchange totalling approximately five million Swedish kronen, and during recent months Japanese purchases of Swedish goods have been primarily financed through Berlin, persumably through German advanced credits.
The Japanese have been informed recently by the Germans that these credits are now frozen and are to used only to purchase of German goods. According to our informant it is possible that this action by the Germans was taken in anticipation of Japan's withdrawal from the Axis. According to another soure it is believed that since Sweden is heavily indebted to Germany for armaments, the Germans are now demanding in return from Sweden goods instead of Swedish kronen. Reports are current here that Japan is now eighty million marks in debt to Germany.
In actual fact Japan is now in virtually the same embarrassing position from the point of view of international finance as are designated foreigners in this country.
这里，作者直接提到了，根据十分可靠的信息来源，日本政府的外汇储备大约是两万德国马克，日本将无力支付外汇合同。由美国，英国和荷兰东印度公司联合制定的冻结条款阻止了日本获得这些国家的货币储备，同时也削弱了日本和南美国家的交易。近几个月以来，日本和瑞士方面的价值五百万瑞士克朗的订单主要在依靠德国的贷款进行支付。不过近期德国告知日本方面日本的贷款也被冻结，而且贷款只能用于从德国购买商品。目前日本已经欠了德国大约八百万马克。这意味着日本的财政即将崩溃。
一些其他有意思的发现（下面的页码都是前面给出的英文原版链接中的页码）：
- 第 326 页: 1941 年，大使已经听到了很多谈论日本将会对珍珠港发动大规模的突然袭击的说法，并且报告了政府。原文是: There is a lot of talk around town to the effect that the Japanese, in case of a break with the United States, are planning to go all out in a surprise attack on Pearl Harbour. Of course I informed our Government.